(ElasticSearch 1.1) Support ES 1.1

* Scalar fields are now single-element arrays
* Removed "Related Content" search
* "Mapping Exists API" now returns empty object instead of error
(ElasticSearch 1.1) Remove manual install of ES 0.90 from .travis.yml
(Elasticsearch 1.1) Added better explanation about search document model
(ElasticSearch 1.1) Split _.extend operations into multiple statements
Fix for intermittent push unit test failures

Bug: The activties were being delivered on the websocket before we had
bound the `message` event listener.

Fix: Bind the event listener in the same process tick as we perform the
action for which we should receive a message on the socket.

The tests have also been cleaned up a bit as they were performing
unnecessary activity collections (push notification happens on routing,
there’s no need for aggregation/collection)
Using `delete` rather than `del` for DELETE rest endpoint

Express has deprecated the usage of `del`. All instances of using the
old method have been replaced.
Fix broken push tests
Update README
Merge pull request #886 from mrvisser/elasticsearch-1.1

(ElasticSearch 1.1) Support ES 1.1
(SearchAccuracy) Don't index publicAlias
(Release 6.5.0) Bump version
(Release 6.5.0) Remove shrinkwrap
(Release 6.5.1) Bump version
(Release 6.5.1) Remove shrinkwrap
Wrapping etherpad html with an UTF8 charset when generating preview images
(Gruntfile) Exclude node_modules of external oae-* modules
(package.json) Revert package.json bump to 6.5.0
Merge pull request #902 from mrvisser/exclude-nodemodules

(Gruntfile) Exclude node_modules of external oae-* modules
(Release 6.5.1) Bump version
(Release 6.5.1) Remove shrinkwrap