Adding the AVATAR_GRAVATAR_FIELD setting

Adding AVATAR_GRAVATAR_FIELD setting to allow developers to define their own user field where the gravatar email is retrieved.

It defaults to 'email', so the behavior will stay the same as currently.

When changed to something like 'gravatar', django-avatar tries to get the avatar in ``user.gravatar``.

This is useful when we want to separate email and gravatar in a project.