Prep for 0.5 release

- Update CHANGELOG and CONTRIBUTORS
- Bump version
- Include Apple Gamecenter highscore setting tutorial in docs

doc-src/tutorials/gamecenter.html

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+
+## The setup
+
+In this tutorial, I'm going to show you how simple it is to creatively
+interfere with Apple Game Center traffic using mitmproxy. To set things up, I
+registered my mitmproxy CA certificate with my iPhone - there's a [step by step
+set of instructions](@!urlTo("certinstall/ios.html")!@) elsewhere in this manual. I then
+started mitmproxy on my desktop, and configured the iPhone to use it as a
+proxy. 
+
+
+## Taking a look at the Game Center traffic
+
+Lets take a first look at the Game Center traffic. The game I'll use in this
+tutorial is [Super Mega
+Worm](http://itunes.apple.com/us/app/super-mega-worm/id388541990?mt=8) - a
+great little retro-apocalyptic sidescroller for the iPhone: 
+
+<center>
+    <img src="@!urlTo("tutorials/supermega.png")!@"/>
+</center>
+
+After finishing a game (take your time), watch the traffic flowing through
+mitmproxy:
+
+<center>
+    <img src="@!urlTo("tutorials/one.png")!@"/>
+</center>
+
+We see a bunch of things we might expect - initialisation, the retrieval of
+leaderboards and so forth. Then, right at the end, there's a POST to this
+tantalising URL:
+
+<pre>
+https://service.gc.apple.com/WebObjects/GKGameStatsService.woa/wa/submitScore
+</pre>
+
+The contents of the submission are particularly interesting:
+
+<!--(block|syntax("xml"))-->
+<plist version="1.0">
+<dict>
+    <key>category</key>
+    <string>SMW_Adv_USA1</string>
+    <key>score-value</key>
+    <integer>55</integer>
+    <key>timestamp</key>
+    <integer>1301553284461</integer>
+</dict>
+</plist>
+<!--(end)-->
+
+This is a [property list](http://en.wikipedia.org/wiki/Property_list),
+containing an identifier for the game, a score (55, in this case), and a
+timestamp. Looks pretty simple to mess with.
+
+
+## Modifying and replaying the score submission
+
+Lets edit the score submission. First, select it in mitmproxy, then press
+__enter__ to view it. Make sure you're viewing the request, not the response -
+you can use __tab__ to flick between the two. Now press __e__ for edit. You'll
+be prompted for the part of the request you want to change - press __b__ for
+body.  Your preferred editor (taken from the EDITOR environment variable) will
+now fire up. Lets bump the score up to something a bit more ambitious:
+
+<!--(block|syntax("xml"))-->
+<plist version="1.0">
+<dict>
+    <key>category</key>
+    <string>SMW_Adv_USA1</string>
+    <key>score-value</key>
+    <integer>2200272667</integer>
+    <key>timestamp</key>
+    <integer>1301553284461</integer>
+</dict>
+</plist>
+<!--(end)-->
+
+Save the file and exit your editor. 
+
+The final step is to replay this modified request. Simply press __r__ for
+replay.
+
+## The glorious result and some intrigue
+
+<center>
+    <img src="@!urlTo("tutorials/leaderboard.png")!@"/>
+</center>
+
+And that's it - according to the records, I am the greatest Super Mega Worm
+player of all time. 
+
+Curiously, the top competitors' scores are all the same: 2,147,483,647. If you
+think that number seems familiar, you're right: it's 2^31-1, the maximum value
+you can fit into a signed 32-bit int. Now let me tell you another peculiar
+thing about Super Mega Worm - at the end of every game, it submits your highest
+previous score to the Game Center, not your current score.  This means that it
+stores your highscore somewhere, and I'm guessing that it reads that stored
+score back into a signed integer. So, if you _were_ to cheat by the relatively
+pedestrian means of modifying the saved score on your jailbroken phone, then
+2^31-1 might well be the maximum score you could get. Then again, if the game
+itself stores its score in a signed 32-bit int, you could get the same score
+through perfect play, effectively beating the game. So, which is it in this
+case? I'll leave that for you to decide.