update tested LuaJIT version
remove some wrapper classes from the module API and declare some classes/methods final

--HG--
extra : amend_source : 805a574362bf07dea656a4f8a1f2ff5fe0fe1c1e
improve dealloc safety of _LuaObject by declaring it as "no_gc_clear" (requires Cython 0.20)
fix test name duplication in tests
add missing name to module "__all__" list
add doc section on "unpack_returned_tuples" flag
prepare release of 0.21