Add basic implementation of Form\Schema.

Add the basic implementation and test cases for the schema class that
ORM-less forms will use.
Start implementing Form class.

Implement some of the basic features for the form class.
Implement remaining basic Form interface.
Add documentation for Form.
Add documentation for Schema.

Also add the ability to provide only the field type as a string.
Fix docblocks and add typehints.
Fix mistake.
Rename isValid to validate.

This makes it more consistent with Table::validate().
Add method for adding multiple fields at once.
Merge pull request #5416 from cakephp/3.0-form

3.0 - Start ORM-less forms