Node packaging

(Just reopening this under a separate branch to keep things cleaner).

Updated the package so it can be properly used with Node. Also exposed a handful of the top-level validation methods, I feel this would be handy for people wishing to use the validation functions outside of a single statement.

If you're planning to publish this to npm, I'd encourage you to use an all lowercase name (xapi-validator) since that seems to be the going convention in the Node community.