Add LIMIT and ORDER BY to MySQL query grammer for deletes

This addition adds LIMIT and ORDER BY to SQL generated when doing a delete when using MySQL. This is valid according to https://dev.mysql.com/doc/refman/5.0/en/delete.html. This is in reference to my origin issue report at #4040.

I submitted a pull request into master, since it changes a bit how queries are done. It shouldn't cause any issues, and should be backwards compatible, but better safe than sorry I guess. If it should go in 4.1 just let me know and I'll make another request for that.

An example of what this would help accomplish is say you have a Post model and you want to delete the 5 oldest posts:  `Post::orderBy('created_at', 'asc')->limit(5)->delete()`

**Note: I accidently deleted my forked repository so I recreated this pull request as I wasn't sure how that would effect the old one at #4106. Apologies.**