Fix gauge support, make the pidfile friendlier

I thought I could send each of these as their own pull request, but GitHub doesn't work that way.  So here's two small patches to pystatsd, cherry-pick them as you please:

1.  Drop the pidfile in /var/run instead of /tmp.  The former is more "standard" than the latter.
2.  Implement gauges on the server-side, rather than having them go through the same code path as counters.  They now adhere to the Principle of Least Surprise when using them.