Add a Command class; add Entry.getFillText()

getFillText() serves to decouple the text displayed in the auto-complete list
via getText() and the text actually filled into the box via getFillText(). This
allows us to display command + args on the list but only fill the command part.

A Command class has been added to provide some structure when extracting the
command name and args. Manually tested and it works.

src/components/views/rooms/MessageComposer.js

@@ -341,7 +341,7 @@ module.exports = React.createClass({
                 MatrixClientPeg.get().sendTextMessage(this.props.room.roomId, contentText);
         }
 
-        sendMessagePromise.then(function() {
+        sendMessagePromise.done(function() {
             dis.dispatch({
                 action: 'message_sent'
             });