Allow floating point times

The experiments we've been doing have been using the `#timing` method to ship floating point millisecond values, and when we switched our code over to using the `#time { ... }` blocks, I noticed our measurements were locked to integer millsecond values and often being rounded down to 0 for sub-millisecond times.

This PR changes the client to allow us to track sub-millisecond performance timing. The spec says "integers", but everything else disagrees, transmitting and storing everything as doubles:

* https://github.com/graphite-project/carbon/blob/master/lib/carbon/protocols.py#L75
* https://github.com/graphite-project/whisper/blob/master/whisper.py#L81

cc @vmg @jssjr @azizshamim 

If y'all approve, I'll package this gem and roll it into a PR internally.