(#9997) Add mysql2 gem support

Besides the mysql gem there is a mysql2 gem that is a "modern, simple
and very fast Mysql library for Ruby" [1]. It can either be installed as a
seperate gem (v0.2.x) for ActiveRecord < 3.1 or can be used as part of
ActiveRecord 3.1

To use mysql2 the dbadapter setting must be set to "mysql2" and this patch
adds support for this setting.

[1] https://github.com/brianmario/mysql2#readme