Tweak authz code for compatability with NULL security protocol.
Implement basic "syncstore" API.

This commit introduces a "syncstore" storage backend build on top of
the kvstore interface, a "syncstore" group of routes for interacting
with it, and corresponding testcases for each.
Consistently use lower-case header name throughout.

This is in keep with the behaviour of the node http server, which
lowercases all incoming headers.
Merge pull request #26 from mozilla/v0-storage-protocol

Implement simple syncstore protocol