mysql@2.x compatibility

The node-mysql client object is deprecated in v2.x and has been replaced with a lower level connection object.  The connection object does not handle disconnects automatically, so the MySQLStore must be able to instantiate its own connections when not using a pool.

I've added a peer dependency on the node-mysql module and updated the MySQLStore to load it (if necessary).  The configuration for the connection (or pool if one isn't provided) is passed in as a `config` option.

These are breaking changes, but the existing master branch fails on disconnections if used naively alongside the latest node-mysql version.