fundmental refactor to allow for pluggable back-end modules

Hey Shawn, 

I'm submitting this as a feature branch because it's a pretty radical refactoring of Graphios.  I did this to make Graphios support pluggable back-ends for various visualization systems. I've implemented four back-end plugins; one each for carbon, statsd, and metrics.librato.com, as well as an stdout back-end that just prints metrics to standard out.  The default install disables everything except carbon.

I've lightly tested this on Nagios4 with the ping and check_tcp plug-ins and although it's running well for me (https://metrics.librato.com/share/dashboards/6ujdizvn) it would not surprise me if there were a few bugs yet to find. I'm happy to rebase/re-pr this in whatever way makes the most sense for you, just lemme know. 

Thanks

-dave