Add support for em_mysql2 adapter

Nice work on this gem, it's been very handy over the years!

I've added support for the [em_mysql2](https://github.com/igrigorik/em-synchrony/blob/master/lib/active_record/connection_adapters/em_mysql2_adapter.rb) adapter provided by [em-synchrony](https://github.com/igrigorik/em-synchrony).  I tried to follow the pattern established by the mysql2 adapter as closely as possible, so the em_mysql2 adapter is just a thin wrapper around the mysql adapter.  I'm not an em-synchrony expert by any means, but I didn't think of any reason why it should work differently than mysql/mysql2 for this library.

I've run the tests for mysql, mysql2, postgresql, and em_mysql2.  All of them pass.

Comments and suggestions welcome.