Default gravatar

Change default gravatar from 'identicon' to:

![default for users without gravatars](http://media.readthedocs.org/images/silhouette.png)

This image is already hosted at http://media.readthedocs.org/images/silhouette.png

Modified the user's profile page to make the gravatar a link that points to https://secure.gravatar.com/  so users can easily change this image. This might be helpful for the occasional user who isn't familiar with gravatars.