The callback passed to .end method receives 3 arguments: err, data and
meta. Therefore, we don't need to capture meta in the serviceMeta
array as we used to do.

- Remove unnecessary setImmediate (since we started using promises, we
are not blocking the main thread anymore).

- Move _executeRequest to Request class (since it operates quite
heavily on the request object).

- _executeRequest always resolves now. The resolved object contains an
err, data and meta properties. This change allows us to simplify .end
and ._captureMetaAndStats methods.

- _captureMetaAndStats is simpler now, it takes the err and meta
object directly so we don't need to guess if they exist or not.

With this change, it's now possible to use fetchr fully
callbackless. If any resource handler is defined with one or less
arguments, we expect this resource to be async and we pass all the
arguments in just one object to it (which also simplifies the
signature of the handlers).