API Fixture Proxy

Intended to replace PR #375 

I create a module called [API Fixture Proxy](https://github.com/quandl/api-fixture-proxy). It attempts to solve the fixture/api stub functionality in a different way. Instead of choosing between a proxy or a stub, it combines them into one. If you want stub, just files into fixtures directory. It will proxy if it doesn't find corresponding files in the fixtures directory.

It works perfectly with Testem (except I had to fork it and hack testem to allow us to hook into their Express server) Testem is working on a middleware interface that we'll be able to when its available. 

Let me know what you think. If you like it, I'll update the documentation.