Add UDP support

I had a need to send metrics to Graphite (not StatsD), so I extended simple-graphite to include UDP support. The new features should be drop-in and backwards compatable.

This included creating a new subclass (<tt>Graphite::Socket</tt>) to support the differences in connecting to <tt>TCPSocket</tt> vs. <tt>UDPSocket</tt>, as well as defining default port configurations for each protocol. I also updated the Rspec tests to understand UDP mocks and UDP operations. While I was in there, I reworded test names a little; if that's a problem, mea culpa.

The work in Graphite::Socket is based in part on work from the MIT licensed gem
"logstash-logger", by David Butler.