Create resources without knowing a subject - signatures as identifiers of a resource

[joepio]

Currently, users have to know the subject of a resource when creating a commit. This is fine for editing existing things, but what if the resource is new? This introduces two problems:

• the client needs to know if the url is available. Maybe the url is taken, or the server does not allow using the url / path.
• the resource is tightly coupled to that server, which means if the server goes offline, the data is irretrievably lost. No real p2p support. Relates to Decentralized persistence and resolving #112

If we use the first signature as the identifier of the resource (or maybe the content hash?) we can fix these issues.

[AlexMikhalev]

I think we need to support both - hash-based URLs for new resources and human-readable one, also it will be good use case for creating own URL shortening service - where atomic.data/hash can lead to atomic-data.dev/full_human_readable_url