Specify graphite output protocol (tcp or udp)

This pull request allows users to specify udp instead of tcp when using the graphite output option. I've tried to copy the style of the rest of the code. Have tested on a couple of boxes and it works for both tcp and udp connections.

We use udp almost extensively as this scales a bit better.
So as to restrict the possible entries to 'tcp' or 'udp', you should specify ```type='choice'``` and supply the choices with ```choices=('tcp', 'udp')```. Here's the documentation for the choice type: https://docs.python.org/2/library/optparse.html#standard-option-types