Simplify zsh custom folder setup

- Use ZSH_CUSTOM env var to specify the custom oh-my-zsh dir
- Add oh-my-zsh custom dir to ~/Dotfiles, and add certain plugins as
  git submodules
- Remove the oh-my-zsh package from the dotfilesrc
Update gitconfig

- Remove "Jobs..." from WIP alias
- Set default git editor to vim instead of vimwrapper script
Update gitconfig and add checkoutmanager.cfg

- Add checkoutmanager.cfg which currently only has one repository,
 ~/bin
- Create diff/merge wrappers which launch the approriate tool
- Configure gitconfig to use the diff/merge wrappers
Update vimrc's MySys function to be more platform neutral
Updates to zshrc

- Remove jira and virtualenvwrapper from plugins
- Remove ~/git/dev-scripts from fpath
- Enable platform dependent settings
-- Enable command-not-found plugin on linux
-- Enable brew plugin and enable zsh-completions on mac
Update zsh_aliases

- Do not alias "ls" to "ls --color" on macs
- Use vim completions with vi