Env vars should not be used outside of the Rakefile

This makes the api class something you actually make an instance of instead of just a static class.  @patrick-wright suggested this change while reviewing https://github.com/opscode/ec-metal/pull/87

Ping @oferrigni @hosh
That was my first thought, but it looked like trying to render a .erb was somewhat non-trivial.  I'm happy to loop back on it, but it didn't seem _too_ awful this way.