Improvement to Git dotfiles.

New user-wide gitignore file to handle common cases such as Vim swap files, Mac
OS X crap, etc. It needs to be symlinked to ~/.gitignore to work out of the box
with my gitconfig file.

This diff contains also some improvements to gitconfig: `git pull` does a
rebase instead of a merge by default, `git push` pushes only the current
branch, and a global gitignore file is looked up under ~/.gitignore.
vimrc: line length now 79 instead of 80.

I'm not sure I like this change, but as it is the standard advised by PEP8, I
thought it'd be better. Also, this diff enables simple folding in vim based on
indentation.