WIP: Add host.exp.exponent (Expo)

This adds the Expo app, which is used to run apps in React Native for Expo. Continued from this forum post and RFP.

Quite a bit was needed to get this build properly without closed-source dependencies. For readability, I haven't formatted the prebuild and build commands (yet, I suppose - that is also the reason why the lint CI is failing). I'm not really sure if it's a good idea to have so many changes, but alas, what to do. Curious to hear feedback.

This app provides integration with expo.io's cloud service (not sure if server part is OSS), but the integration is a bit of an add-on to the app: the main screen doesn't need it. Please advise if this would need an extra AntiFeature.

Pending:

• is it a good idea to indent commands in metadata file, or will clarity be lost?
• check source tarball to make sure all binaries are indeed removed
• get some image assets locally instead of from cloudfront/aws at run-time
• don't contact exp.host at start
• get rid of included jars
• remove key placeholders from manifest template
• get node 7 on build servers (yes, that is required)
• figure out how to make sure that sources downloaded in prebuild remain