Add prototype code for Measure change tracking
Simplify change fetching for MVP version

Will just be tracking associated Measure changes for Commodity
Add change tracking to Chapter, Heading and Commodity
Extend Oplog functionality

Adds Operation#record to allow accessing the resulting record of
particular operation

Also wraps #changes to ChangeLog
Remove TODO note

Used to be an error in ChiefTransformer, this is not used (or needed)
any more
Specify what gets included into Goods code changes
Add views that generate change feed for Commodity/Heading/Chapter
Expose oid with change

Useful for generating unique ATOM entry ids
Make change count a named variable for clarity
Merge pull request #108 from alphagov/goods-code-changelog

Goods code changelog

https://www.pivotaltracker.com/story/show/43647015