Initial object skeleton.
Add PHPUnit skeleton for testing.
Exclude Vendor libs from Git.
Improve documentation.
Make the pretty flag on asJson() optional.
Return offsetGet() by reference so that nested array assignment works.
Add a lot more tests.
Throw an exception if a required property is not set.
Add docblock.
Exclude optional but specified properties if they are not specified with a non-empty value.
Add basic support for XML serialization.
Add support for nested values the XML output.
Add support for parsing incoming problems from JSON.
Add support for parsing a Problem object from XML.