New option to disable user creation via web
Add documentation for new config option
Fix docs, remove deprecated comment
Don't exempt sysadmins
[#1226] Add tests
Merge branch 'master' into 1226-user-create-option

Conflicts:
	ckan/logic/auth/create.py
	ckan/templates/header.html
[#1226] Return the old behaviour for create_user
Fix a few small mistakes in the tests
[#1226] PEP8
Merge pull request #1226 from okfn/1226-user-create-option

Disable user registration with a config