move __version__ into anyvcs/version.py

This is probably a good idea for down the line stuff. This way __init__.py is
free to import stuff as it pleases and won't run into any 'chicken and egg'
issues with setuptools.
read version information from anyvcs/conf.py
reorganize and rename git, hg, svn specific pages

All the pages are linked directly by the index so the documentation is less
deep. The section headers are English instead of module names.