Added support for metric collection for multiple mysql instances

First, thanks for this awesome tool. It's been incredibly useful.

We had two issues with the mysql coffee script:
1) We needed to collect metrics from all mysql instances running on a box
2) If mysql is not running, hoardd shouldn't exit with an error

Both of these have been addressed in this commit.

Sample output:

DEBUG: hoard.localhost.mysql.<b>3306</b>.general.txBytes 130245 1389338486
DEBUG: hoard.localhost.mysql.<b>3306</b>.general.keyRead_requests 0 1389338486
DEBUG: hoard.localhost.mysql.<b>3306</b>.general.keyReads 0 1389338486
DEBUG: hoard.localhost.mysql.<b>3306</b>.general.keyWrite_requests 0 1389338486
DEBUG: hoard.localhost.mysql.<b>3306</b>.general.keyWrites 0 1389338486

As you can see, metrics between instances will be differentiated between using port number.

EDIT: This has been tested successfully on Ubuntu 12.04.