Don't show gravitar if not available

If a user does not have a email address, the gravitar image will not be shown
Clear the SyncTime table when refreshing, this clears all bookkeeping data so
 it will be as if no data was ever syncronised.
CommitComment: Use gson to process the author object, it will still create an
 object without all the fields (e.g. email) being present.

Change-Id: I37fa0b62e700a5000c08a2430ff3054eb1a0e789