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?
Is it possible to do something like:
```yaml
- path: Memstats/*
```

Nitpick: Any reason why on this line it's "Memstats" and line 11 it's "memstats" ?