Add Gravtastic gem to display a user's Gravatar image.
* Add gem to Gemfile.
* Update models/user.rb to include Gravatastic and to specify image size.
* Update views/users/show.html to render Gravatar image.
Merge pull request #50 from tiger-swallowtails-2014/MM_quick_gravatar_spike

MM: Add Gravtastic gem to display a user's Gravatar image.