send to multiple carbon/statsd servers at once

I was just doing some more testing and wanted to be able to emit to to different carbon servers at the same time.. so I rewrote the statsd and carbon backends to work in a list context.  This also simplifies the configs a bit. 

In this commit I also ripped out most of the json configuration stuff from the librato backend and replaced it with simple comma separated lists.  I left the whitelist in json since someone might want to have a comma in a whitelist RE.