Provide extension version number to Sphinx

[MinchinWeb]

In particular, the version number will show up in the Sphinx log. So instead of:

# Sphinx version: 1.3.5
[...]
# Loaded extensions:
#   ablog (unknown version) from s:\documents\github\ablog\ablog\__init__.py
[...]

you get:

# Sphinx version: 1.3.5
[...]
# Loaded extensions:
#   ablog (0.8.2) from s:\documents\github\ablog\ablog\__init__.py
[...]

[MinchinWeb]

:+1: