BibDocFile: pickle support fix

* Fixes errors when trying to pickle BibDocFile objects due to the
  weakref introduced in 4acc073172179038db21ec561e7f4fdf57fceec5.

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Merge pull request #79 from egabancho/bibdocfile-pickle-support

Bibdocfile pickle support