adding json-path, removing implicit casting to JSON pointer

Since we're making breaking changes anyway (i guess this will have to be a 1.0.0 release, anyhow), I figured I'd get [JSON-Path](https://github.com/s3u/JSONPath) in as well. It's a more-verbose superset of Pointer with  embedded javascript... it actually does get `eval`d, which is scary, but since this is node-side (and it runs in a `require("vm")` instance, I think it's worth it.

Probably still missing an example in the README of using Path, but there is test coverage.

Since it now supports 3 pidgin languages (`old > new`, Path and Pointer), i reverted the basic case to just be a direct field lookup, and not a Pointer. You now have to start a Pointer with `/` and a Path with `$.`.

Looking forward to you kicking the tires... let me know what it needs before hitting 1.0.