Add support for 'npm'

Moin,

this adds support for building npm packages.
Simple recipe would look like this:
```ruby
class Less < FPM::Cookery::NPMRecipe
  description 'Less - leaner css'

  name     'less'
  version  '1.6.3'
  depends  'nodejs'
end
```

Cheers,
Jan

PS: Thanks to @mlafeldt.