Updated schema

I've updated the schema slightly, keeping tracks of citations in different tables rather than just keeping one citation field in the reference table. This way, the number of citations over time can be stored (so the rate of increase of citations can be recovered, as well as the number at any point in time). For right now, "citations" will be the number of hits in Yahoo for the title, year, and lead author last name of a reference, since ISI and CrossRef both require payment to use for citation info and Google Scholar's terms of use prohibit display of info from that site (see Rod Page's post about this).