v0.3.0

• made sure that boolean statements filter parameters are passed as the strings 'true' and 'false'
• added missing return statements to some methods of the StatementsFilter class to ensure the fluent interface
• changed the default value of the display property of the Verb class to null (was the empty array before) to make it possible to distinguish the empty list from the case when the display property was omitted
• added the possibility to attach an IRL to an activity definition that acts as a reference to a document that contains human-readable information about the activity
• all values of an activity definition are optional, pass null to omit them
• all values of a result are optional, pass null to omit them
• throw an exception when not existing document data is accessed instead of failing with a PHP notice
• all values of a score are optional, pass null to omit them
• values passed to the constructor of the Score class are no longer cast to float