Support JS defer/async attributes

* Changed the cachebuster $mediaQuery var to $fileAttr, which now supports async or defer attributes for JS files.
Support JS defer/async attributes

- Changed the cachebuster $mediaQuery var to $fileAttr, which now supports async or defer attributes for JS files.
- Removed private host info
Update README.md for JS async/defer documentation
Merge pull request #1 from rediris/master

Enabled support for JS asynchronous loading