Expose server-started and server-failed events

[briandealwis]

We recently introduced an open-browser-on-launch feature in Cloud Tools for Eclipse. We had to add some code to look for special start up strings found in the log to detect when the server is running as it can take anywhere from 1 to 10 seconds for the server to enter a launched state. This same logic is already being done in appengine-plugins-core's `CloudSdk and it would make sense to expose this as another set of listeners.