Proper dirty tracking in record updating

Fixes an issue of setting an attribute to `nil`
Added `has_one` association
DRY'ed association data records tests
Added reflect_on_association for Nested Form and ActiveRecord

closes #19
Added `load` and `find` callbacks
Clarified `belongs_to` [ci skip]
Updated the database_cleaner dependency
after_load and after_find now accept entity instead of criteria
No longer needs the DatabaseCleaner monkey-patch
Added `before_init` and `after_init` callbacks
Repository catch-all delegation
Added `offset`
Added support for Kaminari pagination
Rubify code in README
No need for Repository::Collection
Fixed typo in readme
Release 0.70.0.beta2
Ensure that namespaced Entities get a correctly generated table.
Attribute spec more clearly uses different data for different expected outputs.
Small code cleanups