Use a connected socket to reduce DNS lookups

This parallels WoLpH/python-statsd#21

I also took the time to remove the comment that was in `_send`, since this should address the intent there. And also to use a bit less string manipulation. It shouldn't matter unless someone has an obscene amount of tags, but...

We were also thinking of experimenting with adding a buffering argument to `__init__` to optionally specify a buffer to use for multiple-metric buffering. Would that be something you guys would be interested in?