[NavigatorIOS] Allow translucent on NavigatorIOS

Summary:
Hi,

I've updated the NavigatorIOS component to allow setting the translucent property.

usage is:
```
            <NavigatorIOS
                translucent={false}
            />
```

This is my first contrib to react-native, so apologies if I've missed something.

Cheers,
Owen
Closes https://github.com/facebook/react-native/pull/1273
Github Author: Owen Kelly <owen@novede.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

React/Views/RCTNavItem.h

@@ -22,6 +22,7 @@
 @property (nonatomic, strong) UIColor *tintColor;
 @property (nonatomic, strong) UIColor *barTintColor;
 @property (nonatomic, strong) UIColor *titleTextColor;
+@property (nonatomic, assign) BOOL translucent;
 
 @property (nonatomic, readonly) UIBarButtonItem *backButtonItem;
 @property (nonatomic, readonly) UIBarButtonItem *leftButtonItem;

React/Views/RCTNavItemManager.m

@@ -24,6 +24,7 @@ RCT_EXPORT_MODULE()
 RCT_EXPORT_VIEW_PROPERTY(navigationBarHidden, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(tintColor, UIColor)
 RCT_EXPORT_VIEW_PROPERTY(barTintColor, UIColor)
+RCT_EXPORT_VIEW_PROPERTY(translucent, BOOL)
 
 RCT_EXPORT_VIEW_PROPERTY(title, NSString)
 RCT_EXPORT_VIEW_PROPERTY(titleTextColor, UIColor)

React/Views/RCTWrapperViewController.m

@@ -81,6 +81,7 @@ RCT_NOT_IMPLEMENTED(-initWithCoder:(NSCoder *)aDecoder)
     UINavigationBar *bar = self.navigationController.navigationBar;
     bar.barTintColor = _navItem.barTintColor;
     bar.tintColor = _navItem.tintColor;
+    bar.translucent = _navItem.translucent;
     if (_navItem.titleTextColor) {
       [bar setTitleTextAttributes:@{NSForegroundColorAttributeName : _navItem.titleTextColor}];
     }