Pull shared XHR shim out, add option to static Js module method.

* Moves shim to data/js/network/xhr_shim.js
* Add some yardoc comments
Fix some bugs in my edits, add a spec.
Prevent xhr shim from leaking.
Rename :use_xhr_shim to :inject_xhr_shim.
Fix spec to refer to postInfo().
Merge joev's PR for Rex::Exploitation::Js::Network