osxbundle: split and optimize bundling script

Move the code that copies the dylib's to the bundle to a new script
(dylib-unhell.py) which is called by osxbundle.py.

dylib-unhell is about 20x faster than the previous implementation. This is
accomplished by removing superflous shell-out operations which are kept track
of using an in memory tree of all the needed dependencies. Moreover the
shell-outs have been further optimized by not requiring a complete shell for
every operation and just using subprocess.call (which is equivalent to Popen).