Keyspace extension

[metasean]

Moved from core repo to extensions repo. See amark/gun#62 for details.

Re: amark/gun#60

"Create an extension that only has one key, like 'keyspace' which just uses a regular GUN node where the "keys" are now just fields on that node, that reference the actual node. These fields have the conflict-resolution sync guarantees on them already, so if you want to change where the field key references, you just update it to point to the new node. This then is the overwriting behavior, with the additional perk that it has the eventual consistency feature, which keys don't normally have."