Add new port www/py-libsass

A simple Python extension module sass which is binding Libsass
(written in C/C++ by Hampton Catlin and Aaron Leung).

WWW: https://pypi.python.org/pypi/libsass

PR: 216961
Submitted by: yuri@rawbw.com
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D9531