Use custom user model in tests
Merge branch 'master' into test-with-custom-user-model

Conflicts:
	wagtail/tests/models.py
	wagtail/tests/utils.py
	wagtail/wagtailcore/tests/test_page_permissions.py
	wagtail/wagtaildocs/tests.py
Pull all test database options from env variables

Just pulling two and assuming others broke the base configuration I was
using, which uses a different database name and has a password.
Moved parse_filter_spec into a utils method
Renamed wagtailimages/urls to wagtailimages/admin_urls

This is to free up this module for use for the frontend views
Moved image processing code into wagtailimages/image_processor
Added image serve method
Added error for bad filter specs
Added HMAC authentication to image generation views
Added admin view for generating URLs for images
Added very basic URL generator UI
Added tests for filter spec parsing
Added tests for signature generation
Make generate_signature Python 3 compatible
Minor coding style fix
Improvements to URL generator UI
Moved URL generator into separate view
Added single click to select on URL in URL generator
Fixes to Python 3 compatibility
Added tests for frontend serve view and URL generator