Added support for more explicit joins and table prefixed group by fields

I'm not sure how pull requests really work, but I just wanted to submit some minor modifications I made to the mysql query-generator.js.

I was trying to pull in sums of related records that are in a different table but the select queries support for joins and group by statements prevented me from doing so.

This adds a "join" option to select queries that works exactly like "include" but doesn't actually include the models attributes in the query.

I've also added support for table prefixed "group" values. (eg. { group: "Conversations.id" } )