[Dovecot] The sieve_extprograms plugin included in main Pigeonhole 0.4 source tree

Hi,

I think the sieve_extprograms is mature enough for an initial release and that is why I've included the sieve_extprograms plugin into the main Pigeonhole v0.4 source tree. It is still a plugin, but is no longer provided as a separate package.

This should make things easier. It will be released along with Pigeonhole v0.4.0.

Regards,

Stephan.