New plugin 'jme.queue.pipe_command' added.

This plugin works together with the changes to 'jme.rcpt_to.aliases' to provide the ability to pipe an email to an external command via stdin. Inspired by the 'pipe' transport for Exim (http://www.exim.org/exim-html-current/doc/html/spec_html/ch-the_pipe_transport.html) and the need to pipe emails to external binaries for things like mailing list managers.
Wrote documentation for `jme.queue.pipe_command`.

Updated the README too.
Wrong variable name!
Added 'LICENSE.txt'. This work is covered by the MIT license.
We need to call next() for normal aliases too!
Added example config for jme.queue.pipe_command'.

Fixed up the code a little for it.