Upgrade to Rails 4.0.0.beta:

This upgrades to Rails 4 at master and removes all the previous mass
assignment security code, updating to use ActiveModel's
ForbiddenAttributesProtection module.
Bump version
Remove Mongoid::Paranoia. [ close #2433 ]
Remove Mongoid::Versioning. [ close #2432 ]
Cleanup model tracking for index creation/deletion.
Remove multi parameter attributes
Fix the observers being removed from rails core
Cleanup more observer code. [ close #2597 ]
Boolean -> Mongoid::Boolean. [ fix #2648 ]
renaming db:reseed to db:reset
Update changelog
Namespace Railtie initializer and refactor.

[ fix #2659 ]
Fix broken railtie. [ fix #2659 ]
Fixing specs
Removing _loading_revision methods.

As _loading_revision methods were related to Mongoid::Versioning[ #1418 ], we want to remove it as part of #2432
Remove loading revision
Fix time spec
Fix failing specs around callbacks and mass assignment