Add the related documentation to the README. Include a nullU2FStorage
handler to prevent nil pointer dereferences in U2F code.

Backed by memcache, like the U2F challenge storage. The code has been
refactored to abstract the common replicated memcache client away.

Backed by one or more replicated memcached instances.

Introduces the *services_dir* configuration parameter.

Uses the clientutil.Backend abstraction for the user-meta-server
client API.

Useful for partitioned (sharded) services that need to make
partition-related decisions at authentication time.

Starting with counters for authentication requests, split by service
and status.

Still does not send out "new device" emails, but there are hooks to do
so.