Add Gemfile and test files

* Adds a Gemfile to the project, and updates the gemspec to a more modern format. Removes rdoc as a development dependency, though I'm pretty sure that was boilerplate from whatever tool was used to generate the spec in the first place. This supersedes PR #2.
* Adds rspec and a simple integration test (based off of rajatvig@b21b1c0).

This is all scaffolding so I can test and verify that it works with faraday 0.9.