TextInput onSubmitEditing

[chrisglein]

Split out from #2136
https://facebook.github.io/react-native/docs/textinput.html#onsubmitediting
Needs more investigation. Mobile specific feature, is this needed?

[ddalp]

I think this should be mapped to Enter key down on Single line XAML TextBox.

[ddalp]

Change is in.