Bug 1049916 - Create Firefox Accounts smoke tests to validate user authe...

...ntication R=spenrose
Yeah, this makes me kinda nervous, since we aren't listening for `error` events anywhere, it's possible that the child process has exited, and a new, unrelated process has been created with the child's pid.

This is discussed in [the node.js docs on `child.kill()`](http://nodejs.org/api/child_process.html#child_process_child_kill_signal):

> May emit an 'error' event when the signal cannot be delivered. Sending a signal to a child process that has already exited is not an error but may have unforeseen consequences: if the PID (the process ID) has been reassigned to another process, the signal will be delivered to that process instead. What happens next is anyone's guess.

Since these tests will be run as part of the global Gaia marionette test run, we should probably make sure our process handling is as robust as possible. As I said earlier, I haven't touched this stuff in a while; I will find some good resources to make sure we're following current 'best practices'.