Upd work api

Hi Shaun,

I've added the following changes to the work api as per request:

- allow work to be treated as a section in order to traverse the pages belong to articles and chapters.

- allow to work.getleafs() to accept multiple subtypes.  e.g. chapter as well as page

- changed bibId to be of type String.

Could you please review these changes?

Thanks

Shuang