Don't futz with $^W. Import warnings and remove import_warnings

An alternative approach to #427. Use warnings->import when importing Dancer2 to ensure any code using it has the warnings pragma enabled (just like strict and utf8).

Remove the import_config setting and its trigger that mucks with $^W and purge it from the POD. If anyone really wants to set $^W, they can add it to their app or psgi file and be in control of the results.