Empty Gravatar Support

We already added option to disable gravatar

I guess we can tell wordpress to use default gravatar image everywhere

Lets pass default image as `%22%22` 
See - http://smithsrus.com/detecting-when-gravatar-has-no-image/

That will speed up gravatar loading (as nothing will be loaded for users without gravatar)

Note: we may need to add one more option apart from disabling gravatar
