Add Gravatar support for Users.

:fork_and_knife: 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:

```erb
<%= gravatar_for current_user, 72 %>
```

What it looks like:

![screen shot 2014-01-27 at 1 43 30 pm](https://f.cloud.github.com/assets/928367/2012038/76c726f6-8783-11e3-849d-0c4963537ef9.png)

Pivotal Tracker ID: 63206080
https://www.pivotaltracker.com/story/show/63206080