Makes '/bin' directory writable for busybox

If the '/bin' directory wasn't originally writable, that permission gets
restored by tarfile, and thus copying busybox in there will fail with
"Permission denied". We thus make sure it is writable when copying.
Merge branch 'v0.3' into 'fix-readonly-directories'

Conflicts:
	reprounzip/reprounzip/unpackers/default.py
Makes dir writable when uploading directory/chroot
Adds test for make_dir_writable()
Also adds u+x to parents in make_dir_writable()
Adds rmtree_fixed()
Merge branch 'fix-readonly-directories' into 'master'