Fixes to allow moose module to be installed and imported (#220)

* pymoose: unbreak installation under Python 3

Commit 24ebac50 removed the setting
of the suffix of the binary python module. Unfortunately no explanation
was provided…
Anyway, 'python3 setup.py build' and 'python3 setup.py install' stopped
working because setup.py is expecting a file like
_moose.cpython-36m-x86_64-linux-gnu.so, and it simply ignored _moose.so.
As a result the binary module was not installed.

* .gitignore: ignore "moose" only in the top-level directories

Just "moose" would also match python/moose, with predictably bad
consequences.

* .gitignore: ignore VERSION

It's a generated file.

* Restore moose/moose.py

It was removed in 24dea426. No
explanation was provided, but it seems to have been a mistake, because
without this file, 'import moose' has little chances of succeeding,
since __init__.py includes 'from .moose import *'.

* F...