Removing temporary hack to move to user's home when using su.
Improvements to zsh configuration:
 * Unifying autocompletion and history settings.
 * Bumping zsh history size from 1000 to 10000.
 * Only add ~/bin to PATH when it's present.
 * Move from multi-user install of rvm to single user install for root only.
 * Ensure that we can safely switch back and forth between multiple users who have different installations of rvm without getting any errors.
 * Adopting new way of loading zsh autocompletion support for rvm.