Add support for CPAN packages

Example recipe:

```ruby
class PerlLinguaJARomanizeJapanese < FPM::Cookery::CPANRecipe
  name       "Lingua::JA::Romanize::Japanese"
  version    "0.23"
  maintainer "Mathias Lafeldt <mathias.lafeldt@jimdo.com>"

  build_depends "liblocal-lib-perl"
  depends       "perl"

  fpm_attributes :no_auto_depends?   => true,
                 :cpan_perl_lib_path => "/usr/share/perl5",
                 :cpan_mirror        => "http://artfiles.org/cpan.org/"
end
```

Note that I decided to use the new `fpm_attributes` helper to pass any attributes to FPM instead of implementing them in `FPM::Cookery::Package::CPAN`. Not sure what's the preferred way here though.