A chapter on authentication and authorization

This is a first rough version. There is room for improvement and proof-reading.
Add some stuff to make it nice

 - Show the username when logged in
 - Show a "log out" link when logged in
 - Redirect to the index when logging out

and fix some typo on the way.
Merge pull request #65 from BluesFiend/master

Proofreading
Add venv instructions for Ubuntu 14.04

Add instructions on what to do if the virtualenv command fails on Ubuntu 14.04
Merge pull request #53 from kampfschlaefer/chapter_security_authentication_and_authorization

A chapter on authentication and authorization
Merge pull request #66 from eddie-dunn/master

Add venv instructions for Ubuntu 14.04
Made commands display consistent
Added note about source missing from some shells
wording, grammar, clarified some things etc. (chapter strings, ints, lists, loops etc.)
resolving a merge conflict...
Merge pull request #67 from kerstin/master

 wording, grammar, clarified some things etc.
Merge remote-tracking branch 'upstream/master'