Support more of the node.js module loading algorithm

[pkozlowski-opensource]

This is a top-level bug to track progress to better support of node's loading algorithm: http://nodejs.org/api/modules.html#modules_all_together

We won't be able to support core modules but other than this we can capture pretty much the whole algorithm.

The exact task to tackle are:

• [x] - make karma-commonjs aware of package.json - fixed via 55c752a2394b7fe2f3ea851e2341cb436533d2fa
• [x] - make karma-commonjs aware of index.js - fixed via 4f5c0436702b6affbb5647dc4f0182b5057294ac
• [ ] - replace "single root" with the same algorithm to locate packages as node.js is using - see http://nodejs.org/api/modules.html#modules_loading_from_node_modules_folders
• [x] - expose dirname and filename "globals" - fixed via 046bb2b6be58f8e758724bfc4d6dcfe12de06923
• [ ] - make sure that the above works with the de-duped node_modules