Bootstrap Navigation Walker class (refs #1)

  * Source: https://github.com/twittem/wp-bootstrap-navwalker
  * remove old one
Use and style bootstrap nav walker (refs #1)
Add category transient flusher (refs #1)
Widget areas for header and sidebars (refs #6)
Widget areas in header and customizable header image (refs #6)
Custom header functionality (refs #6)
Customizer for wp-admin functionality (refs #6)
Merge branch 'feature-6-header-image' into develop

Conflicts:
	functions.php