Allow opt-in for verbose output

This PR adds a module-level attribute that controls whether the 'ONLINE MIGRATION' message is output when migrations are ran. The default behavior is updated to suppress this message, and display it if `MysqlOnlineMigrations.verbose = true` is set. 

I've updated the specs and updated the README with the intended usage (in a Rails initializer).

Great gem, by the way. It was exactly what we were looking for. I made this change because the extra noise from the ONLINE MIGRATIONS output during rake db:reset seemed to have little value.

Please let me know if there's anything in this PR that needs changing.

Cheers,

Mark