Turn Gravatars from random helpers into a component

This refactor removes all instances of helpers in templates that call getAvatarUrl:
user_avatar: function(){
    if(author=Meteor.users.findOne(this.userId))
      return getAvatarUrl(author);
},

Now the Gravatars are available as a template straight in the html / handlebars - a more holistic approach a-la Ember.js components.

Just use in any template (without helpers):
{{>gravatar user=this}}
or
{{>gravatar userId=someUserIdToLookup}}

It also takes an optional class parameter (defaults to 'user-avatar').


P.S.
Also includes some lint warning fixes and a smart default to gravatar size.