(ReindexAll) Added principals reindex-all

Complete:
* SearchAPI methods to register handlers
* Generic "iterateAll" helper in cassandra.js
* Principals reindex-all handler
* Global admin REST endpoint to trigger

Todo:
* Unit tests
* Content reindex-all handler
(ReindexAll) Start tightening up for unit tests. Still no actual tests written.
(ReindexAll) Added unit tests for iterateAll functionality
(ReindexAll) Addressing issues with tombstones. Added unit tests for content and principal iterateAll
(ReindexAll) Adding more unit tests for invoking reindexall / reprocess previews through the REST endpoints
(ReindexAll) Self-review + one new unit test
(ReindexAll) Changing format of iterateAll + other review changes
Merge branch 'master' into reindex-all
(ReindexAll) Change Error to an http error object
Merge branch 'master' into reindex-all

Conflicts:
	node_modules/oae-content/lib/internal/dao.js
Merge pull request #367 from mrvisser/reindex-all

(ReindexAll) Added principals reindex-all
Added a content DAO
Issue 251 - lastmodified fix
Re-enabled lastModified test + used constant for visibility.