Rudy/golang expvars

Agent check to report values exposed in go app by expvars.

Expvar will by default expose information about the go runtime and also allow to add custom variables.


@remh Do you have an opinion on the way used to specify what metrics to report?
example with JMXFetch:
https://github.com/DataDog/jmxfetch/blob/master/src/main/java/org/datadog/jmxfetch/JMXAttribute.java#L128-L134