Check spelling only in commit messages.

Inspired by Ingo Karkat's pull request.
Use list:longest for wildmode
Merge python.vim and python3.0.vim into python.vim

Merge Python 3 and Python 3 script versions into
the single python.vim script
Update README
Move python.vim to its own repository

New repository for python.vim can be found at
https://github.com/hdima/python-syntax
ENH: Special highlighting for first (summary) line (length-limited) and following blank line.

Taken from Tim Pope's syntax/gitcommit.vim.