Add PSGI Streaming methods

Add PSGI Streaming methods to CGI::Application

Run modes should be able to return a subref or a file handle, as per PSGI spec.  Implementation inspired by:

http://search.cpan.org/~miyagawa/Catalyst-Engine-PSGI-0.13/lib/Catalyst/Engine/PSGI.pm

with the general idea being: Keep it Simple :)