axe

Add public function descriptions to AXE.c and typedef descriptions to AXE.h
Allow AXEfinish_all to take 0 tasks (no-op), add test for this
Fix race condition in add code (inappropriate use of task struct after
  scheduling it, allowing it to be executed and potentially freed)
Fix very rare race condition in wait_all code (could send signal after more
  tasks were added)
Add new version of fractal test without dependencies or task handles
Other misc. cleanup/improvements

Tested: Ubuntu x86-64 Desktop, jam, koala, ostrich, emu