Iterative parser and support for "ignore"

I rewrote the peppercorn parse function, added support for an "ignore" operation, and added docs. I did this before I noticed someone else had done something similar - https://github.com/Pylons/peppercorn/pull/5 . I humbly submit that my version is better because it's shorter, easier to read, and it doesn't add an unnecessary new exception type.