Support registering admin hooks through decorators
Merge branch 'feature/admin-hook-decorator' of https://github.com/takeflight/wagtail into takeflight-feature/admin-hook-decorator
update docs to use decorator syntax for hooks
update wagtail_hooks across all wagtail subapps to use decorator syntax
Merge branch 'takeflight-feature/admin-hook-decorator'
Return the function again from the hook decorator

The decorator variant of hook registration did not return anything,
meaning that the decorated function would end up being `None`. This was
not noticed, as the functions are rarely called manually, as opposed to
being invoked via the hook.
Typo fix (led to incorrect file beig created).
Merge pull request #470 from takeflight/feature/admin-hook-decorator

Return the function again from the hook decorator
Added requirements file for docs
Use SQLite3 database backend when building docs

This prevents Django from loading psycopg2 when running autodocs on models files
Fixes to documentation of Page attributes
Lots of small tweaks to docs
Merge remote-tracking branch 'torchbox/master' into searchchanges/editorspicks