unix: Initial implementation of FFI module.

Foreign Function Interface module allows to load native dynamic libraries,
call functions and access variables in them. This makes possible to write
interface modules in pure Python.

This module provides thin wrapper around libffi. ctypes compatibility might
be possible to implement on top of this module (though ctypes allow to call
functions without prototypes, which is not supported by libffi (i.e.
implementation would be inefficient))).
Add FFI module example.
Merge pull request #235 from pfalcon/modffi

unix: Initial FFI module implementation