though no class is introduced to read an mbox from stream in remplacement, so
create our own one.

MBOXImporter now have import_mbox(path) in addition to
import_mbox_stream(stream), the former using the std lib mbox class and the
later our own custom class.

This file is generated from doc/email.txt, but for now the generation
process depends on logilab's mkdoc and private xsl files.