Add feature to open image attachments

Images that are attached to messages, either sent or received
can be opened in a new tab by clicking on them.

The previous approach that used Anchors to open the image
attachmets failed in various systems because:
 - Chrome on Windows recognised "blob" as protocol and tried
   to find an app for it
 - Chromium on Ubuntu didn't open a new window to load the URL

The new approach adds a "click" listener to the IMG element and
opens the link using window.open (which seems to be working globaly).

Resolves: #252

js/views/attachment_view.js

@@ -7,11 +7,15 @@
   var ImageView = Backbone.View.extend({
       tagName: 'img',
       events: {
-          'load': 'update'
+          'load': 'update',
+          'click': 'open'
       },
       update: function() {
         this.$el.trigger('update');
       },
+      open: function () {
+        window.open(this.$el.attr('src'), '_blank');
+      },
       render: function(dataUrl) {
           this.$el.attr('src', dataUrl);
           return this;