Link preview processing + square thumbnails.

Special handling:
 * YouTube
 * Vimeo
 * SlideShare
 * Flickr (both regular photos as sets)
#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