add `files[]` for new ender compat to require relative files

hi there,
in love with your promises implementation and it's heavily in use over here at Change.org. We have a node application which purposes its modules on both server and client. We build our client packages with [Ender](http://enderjs.com) and one of the caveats of using `when` is that we can't require relative modules to the package. Eg: `require('when/parallel')`

We are about to cut a release of Ender@`2.0.0` which [solves](https://github.com/ender-js/Ender/issues/181#issuecomment-22612175) this issue but requires package maintainers to list the files which can be included.

Sorry in advance if this is a PitA.