Remove App::Cmd dep and have script/dancer2 bail if it's not installed

[bigpresh]

The other approach discussed on #1602 - just removing the dependency on App::Cmd, so people using older perls can still install Dancer2, and have the dancer2 minting tool exit with an explanatory message if the user doesn't have App::Cmd installed.

Leaves D2 installable for everyone, but with the minting tool not working until they install App::Cmd (until such time as we've replaced our use of that with e.g. MooX::Options).

This is an alternative approach to #1602 and #1603 - I'd expect only one of the PRs to be merged, depending on which we pick.

[bigpresh]

[davidp@supernova:~/dev/github/Dancer2]$ perl -MDevel::Hide=App::Cmd::Setup -Ilib script/dancer2 
Devel::Hide hides App/Cmd/Setup.pm
ERROR: You need to install App::Cmd first to use this tool.

You can do so using your preferred module installation method, for instance;

  # using cpanminus
  cpanm App::Cmd
  # or using CPAN.pm
  cpan App::Cmd

For more detailed instructions, see http://www.cpan.org/modules/INSTALL.html

Without App::Cmd, the `dancer2` app minting tool cannot be used, but Dancer2
can still be used for existing apps.

[racke]

KISS at its best :+1:. Good enough as temporary solution until we replace App::Cmd and the big module footprint it carries along.

[cromedome]

:+1: Thanks @bigpresh!

[cromedome]

I will start assembling a release tonight along with a blog post/email explaining this. Tomorrow I will probably start on the CLI rewrite.