provide easy test functionality

[natebrennand]

3 ways I'm thinking of doing this so far include

1. specific accountSid/token that makes everything redirect to return fixtures
2. providing a drop in replacement that has all the same methods
3. allow users to reset the base url in the client to direct to a local server
   • create handlers that are easily used w/ the net/http/httptest server
   • take a path to a fixture file as the only constructor arg, parse and return for them

If we do 3 right, it could be super useful as you can just spin up the mocked version of Twilio before each call. I think it should also be the easiest one to implement.
thoughts?