Fix issue #473 (Make Pool emit a "connection" event)

The first of these two patches adds a failing test case which demonstrates an issue with the current implementation of `Pool` and its `createConnection` option.

The second patch makes `Pool` an `EventEmitter` which emits the `"connection"` event when it successfully adds an open connection to the pool. This second patch also includes a test case demonstrating the use of and need for something like this (in order to set session variables on pooled connections before they are used).

Cheers!
Marcus