Work with inputNode element directly

Instead of using $scope.command, we work with the inputNode element
directly to have control over the caret position. This let us have
nick completition in the same way as WeeChat.

Fix #74

directives/input.html

@@ -1,6 +1,6 @@
 <form class="form form-horizontal" ng-submit="sendMessage()">
   <div class="input-group">
-    <input id="sendMessage" type="text" class="form-control monospace" autocomplete="off" ng-model="command">
+    <input id="sendMessage" type="text" class="form-control monospace" autocomplete="off" >
     <span class="input-group-btn">
       <button class="btn btn-default btn-primary">Send</button>
     </span>