Remove X-Requested-With header hacks on the incoming requests to preview_on_edit and preview_on_create - no longer necessary, because we're not passing that request to the preview logic

Conflicts:
	wagtail/wagtailadmin/views/pages.py
syntactic sugar for get_page_modes: change it to a property called preview_modes and add a default_preview_mode helper

Conflicts:
	wagtail/wagtailadmin/views/pages.py
rename display_modes to preview_modes for consistency

Conflicts:
	wagtail/wagtailadmin/views/pages.py
Deprecate the `show_as_mode` method in favour of `serve_preview` which additionally takes a 'request' argument
update view_draft and preview_for_moderation to call serve_preview (rather than serve, which for some page models might require additional args/kwargs)
output a DeprecationWarning if get_page_modes has been overridden
Merge pull request #419 from gasman/serve_preview

Use serve_preview() rather than serve() for all preview / moderation / view_draft actions