Keyboard shortcut for open folder

Fix #3950

My first contribution which is a keyboard shortcut to open a folder (starter bug). I have read the [pull request checklist](https://github.com/adobe/brackets/wiki/Pull-Request-Checklist).
 
This isen't a major architectural or UI change so i guess that it isen't necessary to discuss this in the  [brackets-dev newsgroup](https://groups.google.com/forum/?fromgroups#!forum/brackets-dev)? I have also read the [Brackets Coding Conventions](https://github.com/adobe/brackets/wiki/Brackets-Coding-Conventions), my change to the code is the following and i have followed the other keyboard shortcuts in the [keyboard.json](https://github.com/adobe/brackets/blob/master/src/base-config/keyboard.json):

```
"file.openFolder": [
    "Ctrl-Alt-O"
],
```

Code passes jsonlint (instead of jslint) :smile: . I have tested this by simply using the shortcut Ctrl-Alt-O (in OS X Command-Alt-O) and checking that the shortcut is visible in File > Open folder (in the menu bar).

I did run the unit tests (Debug > Run tests). Only one did not pass. I will be back with more information about that specific test. I thought that i could submit this pull request so that you know that it is fixed.