set all envelope fields from mapping

[GoogleCodeExporter]

Related to issue 355.
Option to set all envelope fields from the mapping.

for outgoing:
out.envelope: a dict with all fields in envelope.
- the enveloping process stays the same (group by sender, receiver, etc)
- if messages in same envelope: use envelope of (first, last, one of) messages. 
For now: have to store this 'somewhere' (eg in persist).

Original issue reported on code.google.com by [email protected] on 22 Apr 2015 at 8:20