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
Merge pull request #3767 from cakephp/3.0-conditional-validation

3.0 conditional validation