Changing documentation for validaiton methods to make room for the new changes
Implemented executing a callable for determine if field can be allowed to be empty
Fixed isEmptyAllowed() method
Adding tests for the new conditional checking feature in the validator
Automatically setting the entity provider inside EntityContext

This was required as now the outcome of isEmptyAllowed could change based on
what is contained inside the entity object
Ensuring the right entit is passed as provider in EntityContext
Removing unused property
docs: Remove obsolete references to RequestHandlerComponent::setContent()
Component: Include LogTrait to allow calling log() as in 2.x
Merge pull request #3794 from rchavik/3.0-misc

3.0 misc
Rename SimplePasswordHasher to DefaultPasswordHasher.

This avoids confusion with SimplePasswordHasher class of 2.x which doesn't use
the same hashing scheme.
Merge pull request #3801 from ADmad/3.0-passwordhash

Rename SimplePasswordHasher to DefaultPasswordHasher.
Merge pull request #3767 from cakephp/3.0-conditional-validation

3.0 conditional validation
Update version number to 3.0.0-alpha1