Added "gravatar_email" check to blogs.mak, so users can set their own avatars

I wanted to use my normal gravatar on the participants page, the one I use with github, as opposed to one tied with my RIT DCE, so I added an entry called 'gravatar_email' to my yaml and had the blogs.mak file check to see if that entry exists and, if so, uses that instead of the rit dce.