fixes to buffered client, adds floats and delta gauges

I started using your statsd package. I found the buffered client didn't support %HOST%, so I fixed that. I found there was a typo in buffered gauges.

Then I realized that gauges needed two APIS, one for absolute valued gauges, and one for deltas. So I split the gauge API into the regular gauge (Gauge() and FGauge()) and deltas (GaugeDelta() and FGaugeDelta()). That let me handle negative gauge values correctly.

You can pick and choose whatever you like (especially since the GaugeDelta() change breaks the API --- the existing API is that a regular Gauge() is delta, while a buffered gauge is absolute for + values, and delta for negative values, which is somewhat broken)

-Nicolas Dade