Add check for nil auth callback

I have seen several `EXC_BAD_ACCESS KERN_INVALID_ADDRESS at 0x0000000000000010` in `+[Foursquare2 callAuthorizationCallbackWithError:]` (`Foursquare2.m,`, line 1470). I assume this is caused my multiple concurrent requests and a race condition that causes the callback to be set to `nil` by one thread and then access by another.

Also added an NSAssert for dev build fail-fast, but since NSAssert is disabled in relese builds I also avoided the crash.
Add NSAssert before resetting authorizationCallback
Merge pull request #57 from mzsanford/bad_access_callback

Add check for nil auth callback