Add mysql resource type & be_accessible matcher

This is the first pass at a mysql resource type & be_accessible matcher.

I'm open to feedback on changing the 'be_accessible' matcher name to something else.  Suggestions?

Also, do I need to repeat the provided tests for multiple different distros?  The mysql binary should presumably be consistent across the lot, but I'm happy to update this PR if these tests need to be duplicated for more OSs/distros.