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.
The idea here was that paths would apply to these sorts of names to make them work, eg:

```javascript
System.paths['d/*'] = 'http://site.com/path/*';
```

And if there was no path match, we can't fetch it?

We currently throw in locate if this was the case it has no schema - an alternative would be to move the throwing behaviour to fetch rather @caridy?