Fixes for #244 and #245

[vanboom]

#244 - allow the tracker to track embedded relations

#245 - handle nil original value for an embeds_many relation

[mongoid-bot]

	2 Warnings
:warning:	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.
:warning:	Unless you’re refactoring existing code, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#246](https://github.com/mongoid/mongoid-history/pull/246): Fixes for #244 and #245 - [@vanboom](https://github.com/vanboom).

Generated by :no_entry_sign: danger

[dblock]

Thanks, this needs tests and changelog, please.

[dblock]

Bump?

[johnnyshields]

#244 looks suspect, the line should not be removed. I'll take a look later at this.