Rename Woodchipper-related stuff.

No functional changes introduced by this commit.

Renamed:

    woodchipper.js -> constants.js
    Woodchipper -> ConstantVisitor
    debranch -> propagate
Use recast.parse and .print for require("vendor/constants").propagate.

This removes the need to pass a callback, which is a nice improvement.
Merge pull request #106 from benjamn/rename-woodchipper

Rename Woodchipper to ConstantVisitor, and simplify the vendor/constants.js interface