Etherpad integration
Added the HTTP protocol to the etherpad configuration.
Added the Etherpad API key to the travis file.
Restoring revisions.
Letting etherpad-lite-client point to simong his repo.
Reverting etherpad-lite-client to latest (0.6.0) as it has been made available.
Merged master.
Merge branch 'master' into etherpad2

* master:
  Remove global flag from value regex
  Fixed type of parameter in a doc
  Simplifying skinning variable replacement loop
  Replacing default skinning variables as well as tenant-supplied values
  Making skinning aware of UI hashed file mappings
Merged master.
(PP-refactor) Fixed up some JSDoc + JS syntax
Unit test improvements and minor fixes for the oae-principals module
Updating some basic docs
Merge branch 'master' into etherpad2

* master:
  stub READMEs for oae-modules
  specify deps for preview-processor and activity
  Fix typo, remove unneeded parameter
  Increased timeout when starting tests because of intermittent failures
  (TravisCI) Adding workaround for Cassandra ipv6, fixing test race condition
  (MqCluster) Replicate queues to all cluster members. Better startup/shutdown graceful handling in server.js
  add more supported image types
(Issue 335) Unit test to verify searching memberships of non-existent principals
double image sizes
Documentation fixes
Documentation fixes
Etherpad followup

 * Creating pad/Storing file before creating a content item in Cassandra.
 * Split out etherpads configuration in internal and external hosts
 * Renamed revision.link to revision.downloadLink
 * Cleaned up various comments and jsdoc.
Merge pull request #476 from mrvisser/issue-335

Unit test improvements and minor fixes for the oae-principals module
Added unit test to check that etherpad properties cannot be set on the content object.

Removed internalProtocol from the etherpad configuration.