Add actions for setting TextView text.

Takes care of https://github.com/ReactiveX/RxAndroid/issues/80
Just realized that your implementations are not as robust as the other ViewActions. They all return an instance of ViewAction1 which keeps a weak reference to the view and handles when it is null. The implementation should be like this:

        return new ViewAction1<TextView, CharSequence>(view)
        {
            @Override
            public void call(TextView view, CharSequence text) {
                view.setText(text);
            }
        };