Move all tools into mitmproxy.tools, move models/* to top level

The primary motivation here (and for all the other moving around) is to present
a clean "front of house" to library users, and to migrate primary objects to
the top of the module hierarchy.

examples/flowwriter.py

@@ -1,6 +1,6 @@
 import random
 import sys
-from mimtproxy import io
+from mitmproxy import io
 
 
 class Writer:

examples/redirect_requests.py

@@ -1,7 +1,7 @@
 """
 This example shows two ways to redirect flows to other destinations.
 """
-from mitmproxy.models import HTTPResponse
+from mitmproxy import http
 
 
 def request(flow):
@@ -11,7 +11,7 @@ def request(flow):
 
     # Method 1: Answer with a locally generated response
     if flow.request.pretty_host.endswith("example.com"):
-        flow.response = HTTPResponse.make(200, b"Hello World", {"Content-Type": "text/html"})
+        flow.response = http.HTTPResponse.make(200, b"Hello World", {"Content-Type": "text/html"})
 
     # Method 2: Redirect the request to a different server
     if flow.request.pretty_host.endswith("example.org"):