Adding XDR post support

Mostly for native polling to use the same backbone the rest of the code has, we're now supporting POSTs in browsers that support CORS. 

Since git pulled in node_modules as well, here are the files I've changed:
Adding post_factory, additional parameters to xdr for body, method.
https://github.com/MassRelevance/massrel-js/blob/post-support/src/helpers.js

Updating tests to be more appropriate.
https://github.com/MassRelevance/massrel-js/blob/post-support/test/spec/helpers.spec.js

cc @howardr @acmcelwee @jasontbradshaw