Handle aliased fields with pluck.

[ fix #2867 ]
Autosave should load relation only when necessary

This makes the relation load on autosave happen only if there is changes on the doc or
in one of the auto save associations. [ Fixes #2862 ]
Update changelog
Dont create an $unset for remove_attribute on new.

[ fix #2879 ]
Moving imports into document
Moving around imports
Moving scoping to scopable
Moving reloading to reloadable
Move sharding to shardable
Move dirty to changeable
Move indexes to indexable
Move inspection to inspectable
Moving finders to findable
Move json functionality into serializable module
No need for mongoid namespace
Move serialization to serializable
Add since to serializable
Fix since version on new modules
Move hierarchy to traversable
Get rid of remaining observer code