disable auto js extensions

Here's a prototype for https://github.com/ModuleLoader/es6-module-loader/issues/218.

* In normalize, we normalize relative to the parentName, and when none is provided we normalize relative to the baseURL.
* Paths are applied in normalize, before normalization, only on names that do not start with "scheme:", "./", "../", "/".
* Locate does a final normalization relative to the baseURL

All tests passing.

Feedback welcome @caridy.
This is a tough one actually - most AMD doesn't use extensions, so we could still have the AMD layer itself add the extension. In CommonJS we could do something similar assuming the extension is optional. I'm not sure about this though.