[vim] Add some good defaults for ruby files.

Now using space-only indentation, instead of tabs.
[zsh] Fix some key binding issues with iTerm2.

This makes it possible to use option-left/right and such correctly,
without losing characters and breaking the display of the line.
[zsh] Explicitly enable completion support.

This seemed to be needed on zsh for MacOS X. Otherwise, I couldn't
auto-complete git commands.