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?
I thought it was the only way to show something on the status page but I just discovered this function https://github.com/DataDog/dd-agent/blob/master/checks/__init__.py#L469 (better late than never).
Going to change that