Added pool.end()

As brought up in https://github.com/felixge/node-mysql/issues/911

Please let me know if you would like me to adjust...
Thanks very much for the tip. I was just looking at adjusting it, and it looks like Doug edited the commit before merging it. He seems to have removed the link and adjusted the wording somewhat.

See https://github.com/felixge/node-mysql/commit/dc0d6003a6125707c9018f42a6c3623c901ef674