Add BibTexDB references and citations commands

Transparently wraps the ads package.

TODO: I should fully wrap the ads query and ads article objects in
ADSBibDB and ADSPublication classes so that effectively the API is the
same whether an article is derived from a local bibtex file or ADS.
Add ADSBibDB and ADSPub

Idea is to have different means of providing bibliographic data (via
ADS, or in a local bibtex file) that present an identical API to the
user.