[mcb] implement is_active_callback()

Part of https://github.com/lualatex/luatexbase/issues/17
[attr] fix error message
[mcb] add tests for is_active_callback()
[mcb] implement callback_descriptions()

Addresses https://github.com/lualatex/luatexbase/issues/17
[mcb] add test for callback_descriptions()
update news
Merge pull request #18 from phi-gamma/master

address issue #17