Add Gravatar support for Users.

Creates UserHelper#gravatar_for which takes a User as a parameter and an option
size. The method determines the Gravatar based on the user's email. Displays the
user's Gravatar on their profile.

Example use in the view:

  <%= gravatar_for current_user, 72 %>

Pivotal Tracker ID: 63206080
https://www.pivotaltracker.com/story/show/63206080