Fixes IOS-356 Can't see send button in Message Center with white tintColor

Adding a tintColor property to ATConnect. To test, in FeedbackDemoAppDelegate, add:

       [self.window setTintColor:[UIColor whiteColor]];

and see the resulting problem with our UI with a white tint color. To override the tint color, add:

       [[ATConnect sharedConnection] setTintColor:[UIColor redColor]];

which will use a red tint color in our UI.
Merge branch 'Next' of github.com:apptentive/apptentive-ios into Next