Update Calendar.php

Implemented method chaining
Update Event.php

Implemented method chaining
Update StringValue.php

Implemented method chaining
Update RecurrenceRule.php

Implemented method chaining
Update Property.php

Implemented method chaining
Update PropertyBag.php
Update PropertyBag.php

Implemented method chaining
Merge pull request #15 from Rvanlaak/master

Implemented method chaining