Fix Python builtin collisions.

This renames all validator support functions to CamelCase, but adds
backwards-compatible deprecated functions and classes.

As part of this, converted to a package.
Fixed a few references to deprecated functions I missed.

Thanks to @aaronharnly for the review.
Merge pull request #24 from alecthomas/fix_builtins

Fix Python builtin collisions.
Bump to version 0.6.