Non arc compatibility

Hi,

It's infortunate that BButton is not working with Non arc project. I previously made a pull request to the original repo https://github.com/mattlawer/BButton/pull/14 but it seems that it' not active anymore.

I added some conditions to check if arc is enabled or not in order to prevent some bridge cast and a crash on non arc project.

It now works perfectly with both arc and non-arc project.
Hope it will help people.
Regards.

Olivier