Fix finding libraries in OSX

OSX has no ldconfig. Instead we search the user's DYLD_LIBRARY_PATH.

This is a little hacky (especially the filename construction and returning None if we don't find anything), so suggestions are more than welcome.

This should fix Issue #29. The problem there was that find_lib_path was returning None, which was then passed as a parameter to Popen when calling otool. Popen only takes strings as commands so we ended up throwing an exception.