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
Can't use uri_for() outside of a request.

We can't use Dancer::uri_for() outside of a request being handled, so it's no
good here.

Just returning what we worked out as an URL seems to work, but needs more
testing.
Don't append a slash to our prefix for the list.
Add a logout link.

Just for easier testing.
Merge pull request #24 from bigpresh/add_auth

Add auth