StubEvent implementation

This convenience class allows for identifier-only event types to be
easily created.
ID-less addMapping()

For simple applications or those with very few event types.
Add check for previously-used identifiers

While overwriting the intToClass map would work just fine, classToInt
can become corrupted if a duplicate ID is issued
Add javadoc for addMapping()
Javadoc for getClass() and getInt()