Better Placeholder Detection

[CyborgMaster]

Only remove placeholder elements if they don't have classes marking them as angular generated. Fixes #62

[thgreasi]

Hmmm, seems that one of the tests broke.
It might be a bug in the tests though.

[CyborgMaster]

@thgreasi So I've been looking at you're pull request and I like your solution better than mine. Well done. However. Did your solution cause the same test to fail, if so, how did you fix it?

[CyborgMaster]

Closing this pull request in favor of #65

[thgreasi]

@CyborgMaster As you cas see I added some extra tests (3-4) in order to test even the edge case that: two linked sortables exist and their placeholder option is set to a class used by the repeater's elements. Thankfully after some test I managed to have a successfully tested grunt & travis PR. As you already commented, checking the exact class(es) of placeholder.element() is working for now (though, it didn't felt such a good/proper solution, compared to your split+regex+join code).

Once again, thanks for your coop.