Support for aliased fields + README update

Mongoid-History now properly handles Mongoid field and embedded aliases, which are useful for minifying your DB storage. Example:

```
field :n, as: :name
embeds_many :comments, store_as: :coms
embeds_one  :section,  store_as: :sec
```

Previously the embeds cases were triggering "This should never happen. Please report bug"

Updated CHANGELOG.

Also includes a README update for my previously merged commits (#tracked_edits and #tracked_changes)