Adding following API from oae-following sample module
(Following) Following with search, activity and most tests
(Following) Refactored activity routing in a step to make it 'configurable'

* Fixed memberships search indexing for following
* Moved routing to activity-level configuration
* Introduced L1-cached "associations"
* Moved "producer" and "transformer" into single "activity
  entity type" registry
(Following) Fix unit tests and add more for following and activity
(Following) Changes from self-review
Merged master
(Following) Switch to using AuthzConstants.* for visibility and joinable
(Following) Remove validation on start parameter. This isn't done anywhere else
(Following) Adding isFollowing and canFollow to full user profile
(Following) Following up on first set of review
(Following) Adding unit tests for paging of followers and following feeds
(Following) Fixed bad merge where copying tenants was lost
(Following) Followed up on further review feedback
Merge branch 'master' into following

Conflicts:
	node_modules/oae-principals/lib/api.user.js
(Following) Fixing issue with redelivering orphaned single aggregate activities
(Following) Review followup
Merge branch 'master' into following
Merge branch 'following' of https://github.com/mrvisser/Hilary into following
Documentation improvements
Merge branch 'following'