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?
Should https://github.com/DataDog/dd-agent/blob/master/checks/__init__.py#L553 be updated to perform it?
I guess not, to not risk changing the name of an already existing metric.