Update in-browser to sprint 23 master

Stubbed out ``brackets.app`` and ``brackets.fs`` APIs to catch up in-browser branch with master. Fixed issue in ``ExtensionLoader`` to properly configure requirejs. 

Be sure to run ``nativeServer`` node server before loading index.html in browser

```
cd /path/to/brackets-repo/src
node nativeServer/index.js # path resolution is relative to /src folder
```