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 
Maybe `pause`, `play`, `record`, `record_overwrite`, `strict_replay`? Trying to find something that communicates well the semantics behind each option, even if it has less flair :)