Add mailstream plugin

My first plugin.  A couple of things where I'm not sure what is the Right Thing to do:

* I depend on PHPmailer, so I've just included the module directly.  This is ugly, but saves bothering the user with requests to install it on their system separately.
* I've created my own database table, mostly there to translate back from Message IDs to URIs.
* To deal with schema changes, I use a separate schema version number stored in config.  This is independent of the module version number.
* I use thr-parent to set the In-Reply-To header