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
Moving variable declaration to the beginning of the block
More tests for equality operator overloading
Merge pull request #176 from v-for-vandal/lua_eq

Add default __eq implementation for Lua