[zsh] Oh my Zsh!

- Migrate to oh-my-zsh!

  I evaluated prezto too, but oh-my-zsh seemed a bit more easily
  customizable.

  This includes a custom prompt that's fork of an existing oh-my-zsh
  theme, with virtualenv support and simpler code.

- Remove bashmarks in favor of autojump (install from apt for now).

- Use and switch to the base16 color scheme suite, allowing a consistant
  and easily changed scheme between vim and zsh. Using 256-color mode
  for now, pending chriskempson/base16-builder.