Themes in Brackets core

Initial commit.  A few items remain to be done, but themes are now fully functional as a core component so you can start testing this out.

https://github.com/adobe/brackets/wiki/Themes

* [x] ThemeManager goes into core.
* [x] ExtensionLoader has special logic for themes extensions that don’t have JS
* [x] Settings pared down to just your “General” screen
* [x] All of the other loading/scanning code can go away
* [x] Use jQuery promises, sorry :*(
* [x] Settings screen should not use Knockout
* [x] Use new prefs API
* [ ] Unit tests for theme loading and the customizations (scrollbars, etc.)
this should actually be:

```javascript
return new $.Deferred().reject("Theme not found").promise();
```

Unfortunately, that error doesn't give a lot of information (*what* theme wasn't found?). It appears that information is not available in this context. It's not clear to me why getCurrentThemes() would even return an empty value.