Remove SECRET_KEY from checked in settings file.

Regenerate secret key.
Upgrade django to 1.5.x stable

This required some changes to the hyperbo.la app:

* Remove verify_exists -- deprecated in django 1.4
* Remove unused funtion-based generic view -- deprecated in django 1.5
* Remove deprecated in 1.5 django.contrib.markup. Replace with simple
  markdown renderer.
* Update Inheritance query set to catch exceptions
* Update templates to be 1.5 compliant
  * check for existence of pages before fetching them
  * new URL template tag syntax