Adds support for gravatar when user inputs email

Adding `gravatar` flag to `show`, `showSignin` and `showSingup`. By default, `gravatar` flag is `true` but can be disabled by the user.

![gravatar_take_three](https://cloud.githubusercontent.com/assets/419703/4295450/b510b73e-3de9-11e4-9d4b-308a1e56f69e.gif)