Add Mongoid::Boolean to rails generator

[fixes #3325]

Also remove versioning from model generator
Remove custom generator for ActiveModel
Cleanup autosave

Shave some code from it.
Make it thread-safe
Make autosave use the current persistence options
Adding more tests for auto_save
Merge pull request #3327 from mongoid/autosave_clean

Autosave reword
Fix all broken mongohq specs
Adding spec for #3324
Overwrite true on fields when comming from reference relation
Remove more overwrite WARN on tests
Fix extra query on has_one associations

belongs_to binding was doing an extra query when replacing the in memory object

[fixes #2701] [related #2943]
Update changelog
Move @__children cleanup to reset_persisted_children
Fix database configuration when setup through session

[fixes #3335]
eager load part 1

Adding mock preloaders to eager load entries.
eager load: first batch on tests
eager load: part 2

- adding tests
- ensure that belongs_to, has_one, has_many work
- many to many still broken
eager load: part 3

 - use set_relation on eager_load
 - make many_to_many work
:heart: :two_hearts: :green_heart: tests
first eager_load cleanup