Implement `Page.parent_page_types`

Similar to `Page.subpage_types`, but restricts which pages can have a
specific page type as a child.

Useful for Blog posts being restricted to Blog list pages, or similar
things.
Add tests for new `Page.parent_page_types` setting
Restrict child page types when creating them

The list of allowed child page types was restricted, but if you could
guess the URL to create a disallowed page type, nothing would stop you
creating it. Fixes a failing test case.
Force `Page._clean_sub/parent_page_types` to be a list
Add `Page.parent_page_types` to the docs
Replace all calls to clean_subpage_types with allowed_subpage_types

`clean_subpage_types` should not be part of the public API any more, and
`allowed_subpage_types` should replace it in all instances.
Fix indentation of docstings
Fix errors raised with `resolve_model_string`

It now raises `ValueError` for a badly formatted string, and `NameError`
if it can not find the model. Error messages are not translated. Edit
handlers now use the function.
`resolve_model_string` now raises `LookupError`

Django 1.7 raises a `LookupError` when looking for a model that does not
exist. This brings the code in to line with Django. The `LookupError`
from Django is allowed to propagate in 1.7, and a `LookupError` is now
raised instead of a `NameError` in 1.6.

All code using `resolve_model_string` has been changed to catch the new
errors.
Merge branch 'feature/parent_page_types' of https://github.com/takeflight/wagtail into takeflight-feature/parent_page_types
copyediting on parent_page_types documentation
in to -> into
remove allowed_parent_pages / allowed_subpages.

allowed_parent_pages was a (broken) relic of the aborted no-tree add-page interface where you selected page type first and then specify where to put it. In that context, allowed_subpages doesn't make sense because the page you're about to add doesn't have subpages...
add unit tests for allowed_subpage_types and allowed_parent_page_types
Fix ability to pass a class object in subpage_types / parent_page_types
release note for #491
Merge branch 'takeflight-feature/parent_page_types'