Adding option LUASOCKET_UNIX to install unix domain sockets.

To support `unix domain sockets` build an extra step is required. This patch adds this step through a cmake option: '-DLUASOCKET_UNIX=ON'

Only test in: Linux localhost 3.11.7+ #2 SMP Sun Dec 8 12:13:43 UTC 2013 x86_64 Intel(R) Core(TM) i5-2467M CPU @ 1.60GHz GenuineIntel GNU/Linux