Add new Level 'living' plus tests. Fixes #767, #777, #778

I've added a new level 'living' as we discussed but it is **not** the default level when none is provided. So far the only things I've added are the new implementation of Node.compareDocumentPosition and Node.contains. The tests for these are in the new format as requested. 

The tests for the fixes for the DOCUMENT_POSITION_... constants are in the old style to fit with the tests in test/level3/core.js.

Please feel free to ask for changes. I **think** I've added the new level correctly but I could well be wrong! :D