tracking changes to expose compile method directly

This is more of a discussion pull request.  I've made a few modifications to this plugin in order to get it working as expected with grunt-bbb.

__Added tasks/lib/stylus.js__

This allows us to break out the functionality of compiling stylus.  I changed the signature from `compileStylus` to `compile` (for source) and `compileFile` (for files).

__Modified package.json__

Changed the `main` property in package.json to point to `tasks/lib/stylus.js`.  This allows an external developer to simply require and init the plugin to access the methods.