Better hiera integration

This adds hash parameters to mysql::server to allow specifying grants, users, and databases to
create automatically, which works nicely with Hiera's automatic parameter lookup functionality
Comment mysql::server::providers and update README.md

I added a "$" to the parameter examples because the Github syntax highlighting
looks off without it.
Merge pull request #368 from theasci/feature-hiera-integration

Better Hiera integration