Move the 'hooks' module from wagtailadmin to wagtailcore

Conflicts:
	wagtail/tests/wagtail_hooks.py
	wagtail/wagtailadmin/views/pages.py
update wagtailstyleguide and wagtailcore.rich_text to import hooks from wagtailcore rather than wagtailadmin
update hook examples in docs to import from wagtailcore, not wagtailadmin
Merge pull request #411 from gasman/move_hooks_into_wagtailcore

Move the hooks subsystem into wagtailcore