List of all bib keys in the document (and input documents).

def bib_keys(self):
        """List of all bib keys in the document (and input documents)."""
        bib_keys = []
        # Get bib keys in this document
        for match in texutils.cite_pattern.finditer(self.text):
            keys = match.group(5).split(',')
            bib_keys += keys

        # Recursion
        for path, document in self._children.iteritems():
            bib_keys += document.bib_keys
        bib_keys = list(set(bib_keys))

        return bib_keys