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.

This is required since the EmailThread creation logic strongly rely on the order which Email entities are created

The References header may store a space-separated list of values, which was not correctly handled.

The following three main steps are now implemented in three separate
methods:

- database connection: self.connect(appid)
- importer creation: self.create_importer(cnx)
- mbox file import: self.import_mbox_files(importer, filenames)

The command line helper says that arguments should be
<pyro-appid> <mboxfile1> [<mboxfile2> ...]

Therefore pyro-appid should be the first item of the arguments list,
not the last one.