Added the "reset" method call, to set a connection to a defined state

Greetings,

I use redis and I often set various databases for different purposes.  I have modified your code to include a `reset` method call that will accept a block of code, which will run on checkout, and reset the connection.

Test code is included and passes.

My current usage is to use this for setting the redis database to the default '0' database.

Other usages, would be various MySQL settings (set names utf8, auto_commit, etc)

Please let me know if you have any questions!

Thank you!

-daniel