Added nockWith functionality

added nockWith functionality to automatically record and reuse fixtures.

- added lib/nockWith.js
- added tests, tests/test_nock_with.js
- updated `README`

addresses request in #14 
I think it might be useful to take a look at [Ruby's VCR](https://relishapp.com/vcr/vcr/v/2-9-3/docs/record-modes) and another popular recording module for node [Replay](https://github.com/assaf/node-replay#settings). I don't really like either of their naming conventions, i think they are confusing and we can do better. But they have both been around for a while and are pretty popular so I think we should try to have parity with the modes' behaviors.