Improvements for models

#closes #1655
- removed models as parameter for bookshelf-session
- changed to read permittedAttributes from schema.js
- changed updateTags to be executed at saved event
- added validate to execute after saving event
- added test for published_at = null (see #2015)
- fixed typo in general.hbs
Merge pull request #2169 from sebgie/issue#1655

Improvements for models