Test methods for saving belongsToMany with validation errors in the
joint entities
Adding docs for saving functions in belongsToMany
Making Entity::dirty() able to tell if the entity itself is dirty
Don't save entities that haven't changed since last time they were
persisted
Making it possible to append additional links to a belongsToMany, this
assumes that joint information is present
Added ability to control how deeps associations can be saved and what
options are passed to each level
Adding api docs for some methods
Improving docs for save
Improving more doc blocks