Remove growlnotify requirement (OS X)

[tiktuk]

The dependency on growlnotify could be removed by using:

• pygrowl on OS X 10.6 (and probably some versions back).
• GNTP on 10.7+.
• And if Growl is not installed, by the Notification Center on 10.8 (python lib).

I don't think it is any problem to use pygrowl even though it is old as it (as of today) works fine. And the c extension gets compiled completely transparently when installing with pip.

It could be great to have the right requirements installed automatically, depending on the OS. I don't know if there is any prefered way, other than building the list given to install_requires dynamically before calling setup() in setup.py by looking at something like platform.mac_ver().