Changed num_rows to num_rows()

After a simple get the num_rows parameter appears as null. You must access it's data through the method with the same name.
Defining EXT constant

The EXT constant doesn't exists in the actual version of Codeigniter. ¿Has ever existed?
Defining EXT constant

Add EXT constant in case the ORM it's used in CI 3.x
Merge pull request #4 from Patroklo/master

Changes to compatibilize the library to CI 3.x