Consider using mixins for common methods in the Type classes

[mindjuice]

The Type classes have many methods which are identical except for the class name before .prototype.

To keep the code more DRY, it would be nice to have these in their own file and include them as mixins for the Type classes (e.g., TypeString, TypeBoolean, etc.).

Some of the methods ripe for extraction include:

• _getDefaultFields()
• validator()
• default()
• options()
• required()
• optional()
• validate() - The first several lines of this method could call a common mixin method
• checkType() - This may be renamed/refactored soon, so the name may not be right

Alternatively, if mixins are not desired, there could be a Type base class that all other types inherit from.