pass session context to manifest controller for forms that require authentication, closes #168, output a manifest 404 if authentication is not valid or provided.
some cleanup and edits
Merge branch 'manifest_auth' of https://github.com/MartijnR/enketo into manifest_auth

Conflicts:
	Code_Igniter/application/controllers/manifest.php
	Code_Igniter/application/controllers/webform.php