Re-factor the avatar system and support avatar uploads

The user is now able to upload an avatar, and pick one in a list.

This functionality should eventually be abstracted in a community
package but we still need to work on a great public API. We rely on
collectionFS to manage uploaded avatars. We also removed
bengott:avatar which was trying to solve the wrong problem (namely
displaying the avatar, which is as simple as displaying an image), and
not a avatar system as it should be.

Gravatar support is coming (back) soon. We may also want to have a
list of default fun avatars the user can choose instead of uploading
its own one.

collections/users.js

@@ -43,6 +43,7 @@ Users.helpers({
 
 Meteor.methods({
   setUsername: function(username) {
+    check(username, String);
     var nUsersWithUsername = Users.find({username: username}).count();
     if (nUsersWithUsername > 0) {
       throw new Meteor.Error('username-already-taken');