allow use on server (as a package and not just buildPlugin)

Hey, this is maybe more for discussion than a merge-as-is.  I need to compile "meteor jade" (i.e. jade with components, via your package) on the server (and actually [on the client](https://github.com/gadicc/meteor-jade-client) too, but that's another matter).

This PR is a starting point that allows for that, but points to consider:

1. Currently, only `jade.compile()` is provided (it's all I needed).  What else?  We should also allow direct access to the npm package, I think.

1. Where to place server package stuff?  I did `server/jadeServer.js`, but not sure what your preferences are for this.  `lib/something.js` ?  In the root?  Not sure about your preferences here.  Just one file for now but could potentially be more in the future.