Implements Sharer component

This enables users to add sharing to their apps with ease. It was already possible, but it required configuring ActivityStarter with complicated low-level parameters.

Proposal: https://docs.google.com/document/d/1QY2T4_WCJnMxSLzizj6Sz_8ckhRnOeXljjzSrRoN34Q/edit?usp=sharing

Try it out on http://victsou.com/aishare.
Sample app: https://www.dropbox.com/s/p1uamjxpt0kutk3/Sharer.aia