[REVIEW ONLY] Add concat, compile and minify Grunt tasks

* Change `index.html` to rely on compiled `brackets.less -> brackets.css` via `grunt less`
* Change the default grunt build `grunt` to compile less, concat thirdparty libs, compile brackets src
* Change extract config from `brackets.js` module into new `main.js` root
* Minor changes to extensions and ShellAPI to account for new compile steps
* Update `.gitignore`

On my machine, `document.ready` time is reduced from ~2.5s to ~1.5s, largely due to the compiled LESS code.

Still need to update brackets-shell packaging tasks.