Added support for configuring an input accessory view.

[jspeth]

Adds support for using a React Native InputAccessoryView with the web view. See https://facebook.github.io/react-native/docs/inputaccessoryview for an example. You use this exactly as you would use the input accessory view with a TextInput component.

[jordoh]

@jspeth - thanks, this is working great.

One oddity I've noticed is that calling blur() on a focused element from javascript running within the webview will dismiss the keyboard, but won't resign first responder status, leaving the input accessory visible at the bottom of the screen. This seems outside the scope of this PR, but I thought I would mention it since there doesn't appear to be a way to call resignFirstResponder on the webview without adding it.