feature: add support for launching based on anticipated demand

This is probably non-trivial to implement.

Similar to apache "MaxSpareServers" / "MinSpareServers" [1], awstrial could launch an instance and not asociate it with a user until later. In the same way as apache, this would lower the time that a user had to wait.

The flow might look like this:
 * awstrial is configured to (or notices) that it can expect the need for an instance with a given template, and launches an instance
 * user logs in and requests instance
 * awstrial adds this users's credentials to the already running instance
 * awstrial tells the user how to log in.

This would require we configure some 'push' communication to the waiting instance, either a instance-poll or a server push.

Probably complex, but done and configured well, this could basically remove the wait for EC2 provisioning of an instance.

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[1] http://httpd.apache.org/docs/2.2/mod/prefork.html#maxspareservers