modify <!> to accept a "next" parser (makes it more like noneOf)

- also add notFollowedBy to docs
- fix tests
- add $alphaNum
Fix `choice` docs.

`choice` isn't the same as `<or>`. Instead `choice` takes one
`(listof parser?)` which it `apply`s to `<or>`.
Merge pull request #11 from greghendershott/scribble-choice

Fix `choice` docs.
add $alphaNum to docs