Show only basename and git branch in prompt

* Switch order from "[git branch][pathname]" to "basename git branch %".
* Remove noisy brackets.

Example old:

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

Example new:

    dotfiles dc-prompt %
Show only basename and git branch in prompt

* Switch order from "[git branch][pathname]" to "basename git branch %".
* Remove noisy brackets.

Example old:

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

Example new:

    dotfiles dc-prompt %
Use powerline over deprecated vim-powerline
Ruby motion snippets
Git config changes