Fix \nocite{*} not triggering biber task by parsing bcf file for citations

[Ferdi265]

This fixes issue #57, by parsing the .bcf XML file generated during the first pdflatex run. It contains at least one <bcf:citekey> tag when \cite or \nocite is used. This fixes \nocite{*} not triggering a biber run, which doesn't cause commands to be written to the .aux file, but does create an entry in the .bcf file.