Add 3 abstract methods to Cake\Database\Driver

This is for https://github.com/cakephp/cakephp/issues/2602.  This just defines three new abstract methods inside of Cake/Database/Driver that are already being used by the code base (read the issue to see where these three are currently being used).

I copied the docblock of the three functions from where they are currently defined.  I modified docblock for schemaDialect to indicate a more generic return value and to note that it can return null in a specific case.