Depend on rspec-given.
Require nokogiri.
Rename #has_instance? to #present_in?.
Add license to gemspec.

[Closes #26]
Configure rspec to allow widget tests.
Move UnknownWidgetError to Cucumber::Salad module.
Add spec for #has_widget?.
Rename UnknownWidgetError to Missing.
Introduce #has_action?.
Document Widget.present_in?.