Add an implementation of the heap data structure.

A heap is a convenient data structure for implementing a priority queue.

Code from svn/asterisk/team/russell/heap/.

Review: http://reviewboard.digium.com/r/160/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@176632 65c4cc65-6c06-0410-ace0-fbb531ad65f3