New buttons. Follow changed. ntlarc. Show symlinks

- app.js
  - wrap everything in an IIFE (http://en.wikipedia.org/wiki/Immediately-invoked_function_expression)
  - much refactoring
  - Add the time of the most recently received message when waiting on a pipeline / websocket.
  - Make the "follow" functionality specific to each command, instead of for the whole tab.
  - Added buttons
    - "run again": run the same command again, in the same directory
    - "go here": cd to the given directory
  - Added an "rc" file, .ntlarc.js.
  - Changed the hotkeys.  Added "0" to focus the command editor.
  - Fixed the timing of the loading of $HOME (both the environment variable itself, and the directory listing), the hostname, and the history.  In particular made sure that .ntlarc.js is loaded after the directory listing for $HOME, so that if you want to hide the directory browser in the rc file, you can.
  - Added symlink targets to the directory browser.
  - Started on an "ntla" mode in the Codemirror widget
  - Changed how access to "system" objects works in the c and t objects.
- index.html: Added the above-mentioned buttons.  Changed around some of the other buttons.
- server.go: Added the above-mentioned symlink support.