Show only basename and git branch in prompt

* Switch order from "[git branch][pathname]" to "basename git branch" to read
  more sentence-like ("I'm working on dotfiles in the the dc-prompt branch.")
  and keep project name in same place (flush left) when switching branches.
* Remove noisy brackets.

Example old:

    [master][~/dev/thoughtbot/dotfiles]

Example new:

    dotfiles dc-prompt