[Proposal] Adding the ability to use ->comment() on a Fluent object when using MySQL

I know that in #93 it has been expressed that there is not much interest in implementing this ability.  However, as I needed it, I wrote it and have been using it.  Thus if you have any interest in a quick addition, I have availed this pull request to you :)

Note: It is, as stated in the title, only for the ````MySqlGrammar```` class.