Add spec file for NNNetwork

`NNNetwork` is a collection of classes that power networking in Postr. It is
built atop `AFNetworking` and it provides categories for networking, OAuth 1.0
utilities and read later integration classes (supports Instapaper, Pocket
and Readability).  It also includes iOS 6 `UIActivity` providers for these
services that can be used in `UIActivityViewController`s.