Return child process and add optional options to webdriver_standalone.

[t1bb4r]

I've used the following changes to make the webdriver_standalone function a lot more useful.

Node's child_process can't kill children process in an easy way. I found it a lot easier to just pipe 'q' to the webdriver's stdin.

My gulp task contains the following:

var webdriverServer = webdriver_standalone({stdio: ['pipe', 1, 2]}, () => {});
// Run tests
webdriverServer.stdin.write('q\n');