Importable zip files

extra_files can now include a zip file that can be also indicated in
python_path, and it will be importable by the executed code.
Merge pull request #25 from edx/ned/importable-zip-files

Importable zip files
Remove the tmptmp directory as the sandbox user.
Provide full path to rm.
Fixes for /bin/rm command
A test for the /bin/rm fix, though it doesn't demonstrate the failure
Update temp file test.