Expose psgi_app from the top-level:

Getting the PSGI app should not be done directly via Dancer::Runner
because that might change in the future. Although it is available
in the DSL, we still want to make this available through the top-level
without having to import the DSL.
Merge branch 'psgi-app-for-users'
Fix test in skeleton to use Plack::Test:

We have recently removed Dancer2::Test usage in favor of Plack::Test,
but we forgot that we deploy an example test, and that one still uses
Dancer2::Test. Fixed.
Merge branch 'plack-test-in-skeleton'
reflect changes