new feature: preferred storage servers
fix tests
update peers.preferred docs re: Node Keys
Merge branch 'preferred_peers-rebased' into truckee

No change. Just preemptively merging this to make it easier to merge upstream
master if https://github.com/tahoe-lafs/tahoe-lafs/pull/128 is accepted.

Conflicts:
	docs/configuration.rst
update truckee NEWS.rst