Add support for locking resources

[brettatoms]

GET /family/1/lock - return true/false if the resource is locked
POST /family/1/lock - lock the resource, return true/false if the resource is successfully locked
GET /family/1 - with depth=2 return a locked property so we don't have to check for /lock

DELETE /family/1/lock - unlock the resource, return true/false if the resource is successfully unlocked, can only unlock a resource

POST /family/1 - return a 423 Locked if the resource is locked by another user

[brettatoms]

Also see Bauble/bauble.webapp#10