Navigation plugin. Use it in usual mode.
Refactoring:
  add prefix for event names.
  store event names in constants of prototype.
Add to keyboard shortcuts: open and full screen. Small refactoring.
Navigation:
  - changed fullscreen event capture
  - add fullscreen panel
Title plugin: change title of window on image show.
Bug fix: image container was clickable when no images were loaded.
Issue #2: Show help
  - add help element
  - add help plugin
  - include base Pure stylesheet