munkicommon: remove tmpdir at exit

Currently importing munkicommon has the side-effect of creating a temporary directory in /tmp and this temporary directory is never cleaned up. To fix this with as small a change as possible, register an atexit function to clean-up this directory when the importing script exits.

This won't clean-up the directory in the event that the importing script exits with sys._exit or an un-handled exception. The cleanup function could be changed to remove any directory matching /tmp/munki-* if that were desirable.
Merge pull request #393 from russellhancox/patch-1

munkicommon: remove tmpdir at exit