Add dev script to easily test inbound mail.

A tiny script in the `bin` folder of the buildout that can be used to quickly test inbound mail.

Usage: `cat testmail.eml | bin/test-inbound-mail`

(Assumes you got an instance running on port `${instance:http-address}`, a GEVER client called `mandant1` and an omelette with `ftw.mail` in it installed).

@phgross