Internal Metrics Channel

In regards to #81, this is an implementation of an internal metrics channel. The interface looks like this:

```go
verbose := false
mchan := metchan.New(verbose, "https://u:p@metrics.librato.com/...")

func hello() {
    defer mchan.Measure("some.function", time.Now())
    return
}
```

This will buffer and send a complex measurement to Librato's API.