Renamed contentType to resourceSubType
Link preview processing + square thumbnails.

Special handling:
 * YouTube
 * Vimeo
 * SlideShare
 * Flickr (both regular photos as sets)
Merge pull request #403 from mrvisser/mq-reconnect

(MqRetry) Reconnect properly when RabbitMQ temporarily becomes unavailable
Merge branch 'master' into issue-413
Renamed document to collabdoc
Some more renaming to resourceSubType
Merge pull request #421 from nicolaasmatthijs/issue-413

Renamed contentType to resourceSubType
#423 follow-up

 * Moved downloadRemoteContent to oae-previews/util
 * Added testcase for downloadRemoteContent
 * Cleaned up JSDoc
 * Changed handler of how vimeo detects it can handle a url
Merge branch 'master' into preview-processor-improvements

* master: (22 commits)
  Some more renaming to resourceSubType
  Renamed document to collabdoc
  Fix broken build after 'request' module ugprade
  Adding log parameter for safe path prefix
  Updated documentation style
  Added /api/auth/shibboleth to the CSRF excempt paths
  Added test case for 418 status code
  Add 418 error code to Array of codes in oae-rest util
  Renamed contentType to resourceSubType
  iterateAll PR Follow-up: Fixed superficial styling issues.
  Added missing line break
  (CSRFReferer) Fixes from review feedback
  Google/Twitter Picture PR followup:  * Cassandra.constructUpsertQuery filters out keys with null/undefined values  * Created a TenantsUtil.getBaseUrl(tenant) utility.
  Shibboleth Authentication PR Follow up  * Using simong/passport-saml#master  * Made the SP certificate global admin only  * Added some docs  * Styling issues
  Google picture followup
  Shibboleth authentication
  Removed unneeded testcase for issue 320
  Content iterateAll fix.
  (CSRFReferer) Check referer on incoming requests to verify they are same-origin.
  Issue 320 - Ensure that a group or content item always has 1 member.
  ...
Making PP use new resourceSubType rather than contentType.
Added some documentation in default PP retriever.
Fixed regression in flickr & slideshare PPs
Follow up on square Thumbnail issues
Removed line break
(Notifications) Change router callback to take object {'stream', 'notification'} instead of String[]
(Notifications) Emit event when delivering notifications and activities
(Notifications) Centralize principal updates to DAO for more stable cache eviction
(Notifications) Switch api.group to use dao for principal update
(Notifications) Partial commit, starting unit tests. Squash me
(Notifications) Fixed routing with a couple unit tests
(Notifications) Count unread notifications rather than a simply flag
(NotificationsEmails) Starting emails