Default values for columns on table creation

The database types supported by Kirby allow to specify default values for columns. With the update users will now be able to specify those using the Sql::createTable() method. The user is responsible for escaping default values correctly. 

Note: MySQL does not support default values for TEXT and BLOB columns. For these types the default value is simply ignored.
Clean-up
Merge branch 'dweidner-column-default' into develop