Add chruby and chruby-auto plugins

  * chruby.bash loads chruby
  * chruby-auto.bash loads chruby and enables auto-switching
  * add chruby_version_prompt() function for displaying ruby version
  * inspired by https://gist.github.com/rssvihla/6153455
Added custom bash prompt, based on the 'my' theme.

This colors the arrow at the start of the second line based on the exit
code of the last command: green for 0, red for non-zero

This functionality currently fails when the 'fasd' plugin is enabled,
since it is messing with the PROMPT_COMMAND function.
Merge pull request #235 from nwinkler/nwinkler_theme

Added custom bash prompt
Merge pull request #233 from aramprice/master

Add chruby and chruby-auto plugins