Begin to implement Checker framework and rewrite Complex Object types to be concurrent.

MOD Project descriptor by adding Checker framework dependencies and the Checker framework plugin; the project descriptor was also updated to generate additional Javadoc for test classes.

MOD The complex AMF0 type classes (AMF0Object, AMF0TypedObject, AMF0StrictArray, AMF0ECMAArray) to use a new concurrent approach to managing the internal state of the objects. These classes also now clearly differentiate between the case of a field being uninitialized and unused and the previously ambiguous case of an initialized field containing the value null.

ADD AMF0Null class to have a strongly typed replacement for the Java null within the internal collections of the complex type classes.

MOD Update AMFOutputStream to use the now fully concurrent API of the AMF0 type classes.