Allow the user to pass in a function to be applied to metric names

This provides an option safety net for metrics. Users can do their own kind of sanitization if they wish.

I lifted the concept from the java client as I used it at work and liked how it kept me safe :-)

https://github.com/librato/librato-java/blob/master/src/main/java/com/librato/metrics/Sanitizer.java

I did not go so far as to make it default since I did not want to change the existing behavior. I also think the user should be explicit in saying "Yes, I want you to mess with my metric names" 

`s/tz/tiz`

Matt feels lonely if I don't regularly comment on all his PRs, wherever they might turn up.