Replace #build_targets= with something better

[xunker]

The build_targets syntax is currently this:

activate :target do |t|
  t.build_targets = {
    "phonegap" => {
      :includes => %w[android ios]
    }
  }
end

It could be a lot better. I would like to turn it in to something like this:

activate :target do |t|
  t.groups << "phonegap"
  t.groups.phonegap << "android"
  t.groups.phonegap << "ios"
end