Git: Add a git-editor

This is for viewing a diff side-by-side with a change. Only changes to
be committed will take effect (respects -a flag).

[ Testing: git commit ]
Vim: Remove exVim plugin

This just started failing suddenly on Vim 7.4 latest patch.  So, remove
it instead of upgrading since I never used it anyway.

[ Testing: Opened Vim, no errors ]
tmux: Improve configuration by being explicit

Before we were not explicitly calling out unbinding keys for
next/prev-window and where the pane should open up.  This was causing
weird issues on RHEL5.  Updating this fixes the behavior.

[ Testing: RHEL5 before and after ]
git: Include a hidden gitconfig file

Needed for Amazon work.

[ Testing: Put all Amazon stuff in there, ensured not in Git but still affected devenv ]