Calling increment throws exception

Should increment function take a `by` parameter?  I'm using this with resque_metrics and it expects to call `statsd.increment` with `key` and `by`.

I've had to monkey patch their implementation.  Not a big deal, but I see other implementations out there which expect a number to increment by.

Esty daemon
https://github.com/etsy/statsd/blob/master/examples/StatsdClient.java
