Add routes and hooks to an existing app on import.

Allows routes and hooks for larger apps to be split into multiple source
files, but collected together to form a single "app". Resolves #525.
Update named app tests to use Plack::Test
Add EXPORTS section to Manual.pod explaining usage of appname tag.

Also add documentation the import method that is added to each app,
and the 'with' tag it supports.
Note appname import in Dancer2.pm

Also note that config setting at import time is current not implemented.
use psgi_app directly from Dancer.pm
Merge branch 'pr/named_apps'