librato: fallback to (context || options).source

Librato.mapper.track used to be called with itself as `this`, so `this.name=undefined` so it used to pull `.source` from `options` and not `integrations.Librato`.

this fix will fallback to `context.source` if `integrations.Librato` is not set.
i fixed the our docs too, they used to suggest that `context.source` is the best way to go.
 
cc @gjohnson @calvinfo