feat: support index.js for folder modules

This is the first attempt at supporting more of the node's algorithm for modules resolving:
http://nodejs.org/api/modules.html#modules_all_together

I'm starting with the support for `index.js` for folder modules as it allows me to do the basic code restructure that prepares support for other elements (see  https://github.com/karma-runner/karma-commonjs/issues/21).

The idea here is that there is a function (`getDependencyPathCandidates `) that returns all _candidate_ paths in order in which those paths should be tried.  As soon as it is done we try the candidate paths in order and settle for the first module found.