Add a test that checks for missing migrations

If there are any outstanding model changes that have no migrations, the
tests will fail. Migrations *must* be up-to-date before any code changes
are accepted, or before a new version is released. If they are not, it
causes havoc for all users.

The bulk of the test code is taken from the Django 1.7 `makemigrations`
command, and the South `schemamigration` command.
Fix wagtailimage migrations not using AUTH_USER_MODEL

This caused a false-positive when South checked for changes to models
and an AUTH_USER_MODEL was being used.
Move RegisterDecorator model into wagtail.tests.models

For some reason, registering it via a decorator as part of a function
call caused it to register itself in the wagtail.wagtailsnippets app,
which caused a false positive in the missing migrations check. Moving it
to the wagtail.tests.models file means it is registered in the correct
location.
Made missing migration error message more verbose
Add missing migration for wagtail.tests

It only appears in Python 3.x for some reason.
Added parent page field to copy form and template
Allow changing the parent page when copying
Adapted and added tests for copying pages
Update 0.9.rst
Updated deprecation warnings
Merge branch 'feature/migration-tests' of https://github.com/takeflight/wagtail into takeflight-feature/migration-tests

Conflicts:
	wagtail/wagtailsnippets/tests.py
Help develp me link included closed items
reorganise migrations to remove duplicate, and add migration for RegisterFunction which was previously masked due to duplicate test name
Merge branch 'takeflight-feature/migration-tests'
Merge branch 'master' of github.com:torchbox/wagtail
Added another test for #838

This tests that the page is copied to the different parent when a
different parent is selected.
Changelog and release note for #838
Added Timo Rieber to contributors
Merge branch 'pr-838'

Conflicts:
	CHANGELOG.txt
	CONTRIBUTORS.rst
	docs/releases/0.9.rst
Added changelog/release notes for 0.8.4