osx: make use of the 10.8+ user notification center to display growl lik...

...e notifications

- if 10.8, use user notification center, if <10.8, use growl

why:
- Growl is since 2.0 no longer free available (needs to be purchased for ~2$) and i think bitcoin should have at least a 2nd way for users who want to use free software or a free alternative (notification center is includes in 10.8+)