Add _extension() helper

This is an iteration on @Suor's #55:

* Switched from environment variables to command-line args (the way to set env vars in Windows is different) fcb56db
* Fixed browser support (`is_common_js` or `window.process &&`; checking `process &&` throws an error) fcb56db
* Merged in browser test updates from master b90d9e1 (removed references to other dialects)
* Switched from `construct()` to `applyNew()` (both are answers to http://stackoverflow.com/q/1606797/194758, `construct()` is most popular, but I prefer the second most popular with `ctor.bind.apply()` -- it's a little harder to understand, but it seems cleaner to me) 9ce29e9
* Re-ordered extensions functions with top-level `_extension()` at the top 9ce29e9
* Switched from `hasOwnProperty()` to `_.keys()`, which does `hasOwnProperty()` check if necessary and handles IE<9 edge-case 9ce29e9, f595b88