Add gravatar support

[gnrfan]

Gravatars could be used for Google users based on their email. E.g. Github uses gravatar automatically. An small gravatar for the mantainer could be displayed in the project page and a large gravatar in the user profile.

[gnrfan]

Changeset here:

http://github.com/gnrfan/contributing/commit/88ed37335122c66824a1b57c3fd1777d66c2f9df

Made a pull request here on Github.