Release 1.3.0

• Added ability to add and remove PMValidationUnit instances from a PMValidationManager instance using the new addUnit: and removeUnitForIdentifier: methods.
• Added 'enabled' boolean property to PMValidationUnit, which disables validation when set to NO. PMValidationManager will skip units thusly disabled and not count them toward the global validation state.
• General syntactical cleanup
• Updated tests to test new methods.
• Updated README for clarity