Only recursively mix native objects in `object/deepMixIn`
Add `lang/isPlainObject`
Use `object/isPlainObject` in `deepMixIn` and update docs
Only recursively mix plain objects in `object/deepFillIn`
Increment file versions and update changelog
Use `isPlainObject` in `clone` method and update docs
Update spec wording
Merge pull request #5 from conradz/recursive-merge-behavior

Remove "magic" from recursive merging/mixing