buster-ci should shutdown all browsers if the process exits unexpectedly

[GCheung55]

When buster-ci is exited while tests are running, the agents (browser spawned process) is left open.

Subsequent execution of buster-ci is unaware of the agents. When buster-ci runs to the end and exits, the agents do not close and buster-ci exits the process with code 1.

I'm proposing that an exit event handler is attached to the buster-ci process to close agents if they remain open.

Some documentation on the process exit event: http://nodejs.org/api/process.html#process_event_exit

[dkl-ppi]

Closing the browsers on process exit is something i have already on my todo list for buster-ci. I will implement it before version 1.0 of Buster.JS.