Add support for configurable permissions decorator

This adds the setting `STRONGHOLD_PERMISSIONS_DECORATOR`.
`STRONGHOLD_PERMISSIONS_DECORATOR` allows the developer to define the
decorator to use to define the behavior of Stronghold. The current
behavior defaults to the login_required decorator. You could change this
setting to staff_member_required and it would apply this to all non
whitelisted or views marked public.

Developers can also write their own decorator that requires any
combination of permissions they desire here.

Fixes #34 Thanks @andybak for the idea.