Warn when factory tries to re-register an existing class with different bases. Fixes #1409.
add warn paramter and set it to True from lang, when class is re-declared.
Merge branch 'master' of https://github.com/kivy/kivy into warn-kv
Merge pull request #2052 from matham/warn-kv

Warn when factory tries to re-register an existing class with different bases