Add URL.clean to replace backslashes with forward
Use URL.clean for new Script url and Loader.resolve to workaround issue
in FF.
In require:
- Ensure resolved loader paths end in a single forward slash
- Destroy failed script tags
- shift(), rather than unshift() next path (!?!)
- Skip 'allowed' scripts in cycle detection, to prevent infinite loop.
In require:
- Adjust Loader.clone() to use a singleton per path/source
- Add Loader.default for the initial instance
- Add Loader.using for temporary replacement
- Adjust Loader.use() to use a callback to control loader scope.
- Replace Loader.current with more dynamic context.
- Remove Script instance ownership wrangling
   * Replaced by Loader context, and current Script's Loader changes
- Remove Loader id requirement from Script id references
   * Except in allowed[], to prevent cross Loader confusion
Tidy up require
Adjust CommonJS test rig to use new Loader.clone() mechanism
Add unit tests for:
   * lib/require
   * lib/structure/map
   * lib/structure/set
Add testem, jasmine and phantomjs dev dependencies
Update testing section in README
Remove old doc structure
Add testem configuration