Split the jade package

We now have two distincts packages:

* `jade-compiler`: which exports a `Jade.compile` This package is
   currently server-only, but I'd like to support the client as well.
* `jade`: which is just the file handler for the meteor build system.
  This package uses `jade-compiler` as a dependency.

This is a similar architecture to spacebars/spacebars-compiler.

closes #89 #94