Use Faraday as backend to have interchangeable adapters

Hi,

I found your library quite useful to build an API client for a project I work on. I was in need to inject the rack test session into it to test the API with the client to ensure that the client and API stay in synch. To archive this I put Faraday in to have the ability to change the http backend on demand.  

Well it would be lovely if you would accept my contribution. Let me know what you think and if you like any changes. 

Cheers
Thilo