Bug 1013296 - Compose. Change send button to an paper plane icon

I think it would be override by the has-prefer-sim indicator here:https://github.com/mozilla-b2g/gaia/blob/master/apps/sms/style/sms.css#L570
Currently we have before for counter indicator and after for prefer-sim indicator .In subject patch we will put counter indicator into another element, and put prefer-sim indicator to before element in bug 990537. After these changes we can replace send text with icon in after element safely ...