hack in support for overriding the default system provided folder icon

Currently TextMate will defer to `[[NSWorkspace sharedWorkspace] iconForFile` to pick the icon for a folder in the project drawer (https://github.com/textmate/textmate/blob/61bb49ad6e7e24b62a68fa11b1ec674079ac545c/Frameworks/OakAppKit/src/OakFileIconImage.mm#L134)

this PR makes a change such that if a file called `CustomFolder.icns` exists in the project it will be used instead

before:

![screen shot 2014-02-03 at 9 16 03 pm](https://f.cloud.github.com/assets/39759/2073434/c60761f8-8d5b-11e3-830b-e526cb728a8a.png)

after: 

![screen shot 2014-02-03 at 9 18 59 pm](https://f.cloud.github.com/assets/39759/2073436/e5ed66a2-8d5b-11e3-9964-daa39c54110d.png)

This is only my second ever Objective-C patch so I doubt this is implemented the correct way. I'm hoping someone more familiar with TextMate can rewrite this PR to do it properly!