single_backstroke_delete defaults to true

@kenearley @stof @koenpunt @mlettini @starzonmyarmz 

Makes single backstroke deleting the default behavior. Presently, you have to hit backspace twice in Chosen to delete an entry in multiselects. This flips it to mirror the behavior of things like Gmail and Facebook.

I think we should consider ripping out the double backstroke supporting code entirely, but we'll see if anyone complains about this before we take that step.

Closes #639