Use symbols as keys for Metrics and Subviews

Since NSLayoutConstraints doesn't accept symbols as keys for Metrics nor Subviews,
I created this pull request to fix it. it's nothing special only a little rubyfication


``` ruby
Motion::Layout.new do |layout|
  layout.view view
  layout.subviews state: @state, action: @action
  layout.metrics top: 200, margin: 20, height: 40
  layout.vertical "|-top-[state(==height)]-margin-[action(==height)]"
  layout.horizontal "|-margin-[state]-margin-|"
  layout.horizontal "|-margin-[action]-margin-|"
end
```