Implemented ability to specify source for counter or gauge

Hi,

I have implemented ability to specify source for **any value** of counter or gauge as described in [spec](http://dev.librato.com/v1/post/metrics). 

> source and measure_time can also be specified as a parameters outside of the gauges and counters measurement hashes. In this case the given source and measure_time values will be applied to all values submitted unless those measurements have another source or measure_time specified in their sub-hashes.

Could you please check PR and deploy new version to Maven repository?

Thanks