Local installation (z-shell)

This is regarding issue #50.

1. If install.zsh is run as root, it installs globally (old behavior).
2. If install.zsh is run as non-root, quits with error message.
3. If install.zsh --local is run, installs it locally.

When installing locally, defaults to ~/.autojump/ unless overwritten by user using the --prefix option. It puts all files required for autojump in a single folder for easy removal, and outputs relevant ~/.zshrc changes for user to manually edit.

There are also some minor formatting/wording changes.

If these changes garner interest and gets merged into the master repo, some other changes I'd do:

• modify uninstall.zsh to detect and remove local installs
• test if ~/.zshrc has been modified appropriately and adds changes if not
• make relevant tweaks to support z-shell on Mac OS X
• rewrite the bash equivalent install.sh and uninstall.sh files to match.