Nav ios hide hairline

Summary:
For the current project I am working on I needed to add a boolean option (`shadowHidden`) to remove the 1px hairline shadow from the NavigationIOS component:

`<NavigatorIOS shadowHidden={true} initialRoute={...} />`

By default, or with `shadowHidden={false}` it looks like this:

![withhairline](https://cloud.githubusercontent.com/assets/129330/8145401/d2704956-11d4-11e5-86e8-a75435b68480.png)
Setting the shadow hidden  with `shadowHidden={true}` looks like this:

![nohairline2](https://cloud.githubusercontent.com/assets/129330/8145405/148ed56e-11d5-11e5-85d6-f8cd3453d5ac.png)

The code it uses to do the actual hiding is... a bit of a hack (*I* think), but it's the only way currently to do it, and is the way they do it in the iPhone calendar app: iterating through the subviews and removing the shadow image.

This removes the shadow *and* keeps the translucent blurry effect with a ScrollView (see this [SO discussion](http://stackoverflow.com/questions/18160173/how-to-remove-uinavigationbar-inner
Closes https://github.com/facebook/react-native/pull/1615
Github Author: Mr Speaker <mrspeaker@gmail.com>

React/Views/RCTNavItem.h

@@ -19,6 +19,7 @@
 @property (nonatomic, strong) UIImage *backButtonIcon;
 @property (nonatomic, copy) NSString *backButtonTitle;
 @property (nonatomic, assign) BOOL navigationBarHidden;
+@property (nonatomic, assign) BOOL shadowHidden;
 @property (nonatomic, strong) UIColor *tintColor;
 @property (nonatomic, strong) UIColor *barTintColor;
 @property (nonatomic, strong) UIColor *titleTextColor;