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
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).