Jetpack Dependencies: Combine child and parent theme requirements

[laurelfulford]

This came up while investigating a recent issue (#28): it would be great if the Jetpack Dependancy script could 'figure out' what's required from both the parent and child theme, to be displayed in the message on the child theme.

It looks like the only parent/child theme pair Automattic has that has different Jetpack functionality is Superhero/Sidekick, but that could change down the road. This could help make the message more accurate in those cases.