osx: make use of the 10.8+ user notification center to display growl like notifications

- if 10.8, use user notification center, if <10.8, use growl

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Merge pull request #2615 from jonasschnelli/mac10_8_not_center

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