Allow admin to set default gravatar type

This change allows administrators to select the default Gravatar type used if a Gravatar was not found for an e-mail.
If the setting is set to "No default Gravatar" the avatar-default.svg will be used instead.

Here's an example of those Gravatar types (destinee.larsons gravatar was updated before I changed the setting to monsterid):
![2013-06-01_2143](https://f.cloud.github.com/assets/642457/595537/9ba2c8b8-cb09-11e2-93a8-867f6f1cf5e7.png)