Refresh the codebase

This is a major spec refactoring.

I removed VCR and Fakeweb. The former was complicated to adopt in our case (given for example some endpoints cannot be tested for real, such as Certificates). The latter is unmaintained.

I replaced the code with Webmock and some updated fixtures.

I also made a little bit more easy to contribute on the library. You can now download the code and run `rake` with no additional configuration. This should avoid issues such as the one described in #30.