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 
`dont`, `doesnt` should be `don't`, `doesn't`, etc. here and elsewhere

`wild` for mode name doesn't say anything. I think something an antonym of `lockdown` should be used: `open`, `pass`, `passthrough`, etc.

I also think `record` should *always* record, ignoring the already recorded nocks, as that's what it means to record. In my experience it's very common to want to overwrite old recordings. If we don't provide that users will have to manually delete fixtures whereas the tests already know fixture names and can do that easily. Maybe we need to provide 5th mode for "record new don't overwrite the old"