Remove forgotten remnants of Mongoid::Observer

c.f. mongoid/mongoid#3108
Mention removal of Mongoid::Observer in CHANGELOG
Merge pull request #3289 from herimedia/niels-remove-observers

Remove forgotten remnants of Mongoid::Observer
update changelog
mark as pending a IdentityMap broken test
Use ActiveSupport::Notifications to send operations logs
add log subscriber tests
Minor fixes in comments
Another minor fix in comments
Merge pull request #3295 from Spone/patch-1

Minor fixes in counter_cache comments
Point to moped master on Gemfile for now
Merge pull request #3292 from mongoid/log_subscriber_spec

add log subscriber tests
Mongoid4 re-introduced bug #2678 - sum should be 0 not nil

Updated code and spec. Aggregable#sum should be 0 and not nil
Merge branch 'master' of github.com:mongoid/mongoid