Create an http api for incoming data in soledad server

This is meant to be a generic API for delivering incoming data to a user, and Leap MX will be its first user. This issue is just about the creation of the soledad server endpoint, and does not account for how the incoming mechanism actually works. Further work will be handled in subsequent issues.

• add an IncomingResource
• direct requests to /incoming to the IncomingResource
• make /incoming accept PUTs, ignore whatever comes in and reply with a 200.
• map ideas for connecting mx to soledad server and discuss with platform devs.