Add default __eq implementation

* Renamed SWIG_Lua_equal to SWIG_Lua_class_equal
* If class has no __eq implemented, then default __eq is provided.
  Default __eq compares actual pointers stored inside Lua userdata

This pull request is done as response to Eric Wing proposal "[Swig-devel] Proposal: SWIG should supply a default __eq for Lua, and operator== doesn't work in %extend"

A few more tests should probably be added. I am still unsure that this change wont break existing code.