mGerrit: Add gravatar icons

Powered by Koush's UrlImageViewHelper

Patch Set 2: remove unwanted changes (and some autoformatting)
Patch Set 3: remove more unwanted changes, some refactoring
Patch Set 4: credit MD5Helper to gravatar docs

Change-Id: Id54779d72400b35457e1bf5f31b2111ef977a973