First steps towards using DPAE for auth.

Adding options to use Dancer::Plugin::Auth::Extensible to provide authentication
& access control.  See Issue 23.
Using auth stuff in example app
Use _ensure_auth for record list.
Pass the handler coderef when using require_role etc
D::P::SimpleCRUD loads DPAE if needed.

We don't need to load DPAE ourselves here; D::P::SimpleCRUD will load it if
we've specified auth settings.
Update example to demonstrate roles too
Fully-qualified uri_for() call
Document new auth option