[entities] added map for XML entities
[tokenizer] added support for XML entities

also moved handling of trailing data to _handleTrailingData() (as it
has to be called recursively now)
[tests] also test trailing data support in the numeric entity test
[tokenizer] fixed handling non-existent entities
[tests] added test case for XML entities
[tokenizer] added _emitEntity

as a preparation for supporting decoding entities in attribute values
3.2.0