OS X 64-bit support

Hi,

I noticed that the latest version of Skype (6.1) for OS X includes a Skype.framework that's compiled for i386 and x86_64. I also needed to make a change to the CC variable in the Makefile to make it compile on my system (OS 10.8, Xcode 4.6) so I changed it to clang.

I'm not a native code guy so I'm not sure that I made this change in the best way possible... in particular the jnilib that comes out of clang references the Skype framework as @executablepath/../ which caused me some trouble - I had to call install_name_tool in the Makefile to make it point to the right place.

The unit tests don't succeed - calls like getVersion seem to fail - I don't know if this is a result of the changes I made or because Skype 6.1 behaves differently. There may be something more complicated going here that I'm not understanding