Vapniks

Hi Andreas,
                 I've added a new file pdf-navigate.el. See the commentary in that file.
If you decide to include pdf-navigate.el you might want to alter the preamble a bit, or merge the contents into pdf-history.el

Also:
  - pop-to-buffer instead of display buffer in pdf-occur 
  - added support for follow-mode (https://github.com/emacsmirror/fm) in pdf-occur.el
  - make pdf-history-push interactive, and give it a keybinding
  - remove pdf-history-after-change-page-hook from pdf-util-after-change-page-hook