It looks like zipfile module by default uses the permission mask 0,
which after unpacking on Unix-based systems leads to the files being
inaccessible.
We now explicitly set file mask to rw-r--r-- to match .tar.gz defaults.
Fixes#217.
We used to use the current timestamp when building the archive; switch to using
the timestamp of the tag with the version we're packaging.
This requires some monkey patching since tarfile module is always using current
timestamp when writing gzip header...
Also exclude archive.py from archive and simplify release file list in Makefile.
