Bibharvest: new filter for PoS

* Downloads fulltexts and adds the proper FFT datafield

* creates 3 files:
  append.xml for already existing records
  insert.xml for non existing records
  not_found.xml when fulltext harvest fails

* Adds beatifulsoup4 requirement

* Amends bibharvest/Makefile to setup the new filter

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>