Clean up zshrc and add git info/branch to prompt

* Show a right-hand prompt when we're in a directory underneath or
  at the root of a git repository, showing the branch name, as well
  as colored glyphs indicating whether there are untracked files
  present, whether there are unstaged changes, staged changes, or
  a merge in progress.
  This is courtesy Josh Dick -- see the example screen shot at
  <https://gist.github.com/joshdick/4415470>.
* Keep /sbin and /usr/local/bin at start of PATH
* Make ENVPATH more reliable by using readlink, and supporting
  different arguments for specific OS types
* Include new common.sh file for detecting the OS and setting a
  "trusted" OS type name without having to use regex matching
  on OSTYPE. Both zshrc and install.sh use this now.
* Use new source_if_exists function to include other zsh files
* Set alias for Sublime Text on OSX
* Slightly rearrange paths/sources into better groupings
* Initialize rbenv if present