PsychtoolboxKernelDriver64Bit: Try to make it compatible with both OSX 10.8 and 10.9

This build changes the base SDK to the OSX 10.8 SDK and the deployment
target to OSX 10.8.
This should work on 10.8 and 10.9 and has been successfully tested to
work on 10.9.
Tests for 10.8 pending.
-> Xcode project and PsychtoolboxKernelDriver64Bit.kext.zip updated.