Fixes for argcomplete

- separate out most argcomplete related stuff in new file _argcomplete.py
  (could probably be in the py library)
- allow positional arguments to be interspaced with optional arguments
  ( + test in test_parseopt.py )
- removed double argument in tox.ini
- add documentation on installing argcomplete (>=0.5.7 as needed for
  Python 3), might need improving/incorporation in index.

This does not work on 2.5 yet. I have patches for argcomplete
(with/print()/"".format) but I am not sure they will be accepted.
Agreed with hpk not to push for that.

Removing argcomplete and leaving completion code active now works by early
exit, so <TAB> no longer re-runs the programs without parameters
(which took long for py.test)

test calls bash with a script that redirects filedescriptor 8 (as used by
argcomplete), so the result can be tested.
minor adjustment, added test for positional argument completion
Merged in anthon_van_der_neut/pytest/argcomplete (pull request #50)

Fixes for argcomplete