Track more meta info (git revision, build time)

- centralize meta info in the Manifest;
- add more meta info (build date, git revision);
- add a --version (-v) option, to show this info;
- cosmetic: remove tabs in build.xml.
Add -v and -r options to the output of --help
Move common Sphinx python code to a separate module

fixes #18
Parse build.xml to get the version number for the docs

fixes #17
Show build date and git revision in the About window