Support multi level list numbering

Implemented a changed list counter setup where each list level has its own
counter. This is to support behaviour defined where you can display only a
subset of the list numbers.

This is defined here:
http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#__RefHeading__1418890_253892949
Refactor stylesheet error handling

Conflicts:
	webodf/lib/odf/ListStylesToCss.js
Implement continued numbering of previous list

This implements the text:continue-numbering attribute which specifies
whether the previous list numbering should be continued if it has the same
style as the current list.

http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#__RefHeading__1418834_253892949