Binary heap fix & improvement.

Fixed a bug when removing last element of binary heap (its parent would
be removed instead if it had the same value).
Binary heap now allows custom swapper function/macro. Added `swap_ptr`
macro to swap two pointers in place (`swap` is not suitable for
pointers). This allows to store pointers in binary heap.
Merge pull request #64 from piotrhalaczkiewicz/master

Binary heap fix & improvement.