DISCO: add new resource STRINGdb

http://string-db.org/newstring_cgi/show_download_page.pl
Will need a license agreement.

Here's the schema:
http://string-db.org/newstring_download/database.schema.v9.0.pdf

I think we want the following tables:
ALL from the "network" and "evidence" classes (as described in the schema above)