Added support for group by expressions

Hi.
First off I want to say that mechanoid is AWESOME , it really makes your life easier :+1: )

Second  I've been wondering why group by is not implemented in the select/createLoader methods , so here it is. I know that it can be specified in the .mechdb file when creating a view, but it can't be done in a sane way in the code.

Since the contentResolver / loader  does not  support groupBy in the parameters, i'm passing the group by clause with the URI as a query parameter to the ContentProvider.

Drop off a line if you have some critics or comments.

Best Regards,
Rangel