Add support for embedded polymorphic docs
Spacing fix
Merge remote-tracking branch 'origin/polymorphic' into 2.4-stable
Add change_details method for nicer change summaries
use map for building cleaner change summaries
include original fields for nil modified cases
Added spec for nested embedded documents. refs #42
Fixed error in association chain creation. The current node was not used as the object when looking up the parent relations, instead self was used.

Conflicts:
	lib/mongoid/history/trackable.rb
Tests cleanup:
- Cleanup spec helper
- Remove dependency on Mongoid.yml which is not committed (makes it easier to run tests "out-of-the-box")
- Remove database cleaner dependency
- Make RSpec dependency more permissive
Merge remote-tracking branch 'remotes/getaroom/2.4-stable'
Re-adding cascading_callbacks = true to test. There seems to be an issue in Mongoid related to cascading_callbacks + timestamps, so a few cases are commented out. Have raised an issue in Mongoid for this (https://github.com/mongoid/mongoid/issues/3078).
Changing spec models to make clear that embedded docs can be polymorphic and don't need to explicitly reference their parent class name.
Streamline test code as Mongoid no longer has an issue reloading embedded documents
Add #tracked_changes and #tracked_edits methods. #tracked_changes replaces #change_details which was committed only on the getaroom branch.
Cut test run time in half by refactoring tests to use #let, and avoid re-initializing model classes on each run.
Skip edits where both fields are blank
Updating CHANGELOG.md
Merge pull request #65 from johnnyshields/rebase-getaroom

Polymorphic embedded support + test refactor + #tracked_changes and #tracked_edits methods