update webtest.py to set up headless browser driver correctly for OS X environment

[kaptaink]

In my OS X environment I can't utilize the Display() function as it relies upon pyvirtualdisplay which doesn't run properly in OS X.

Instead the combination of installing Selenium on my Mac, Chromedriver, and Chrome Canary. Setting the following chrome options will allow the Chrome browser to run headless (without using the Display()):

• options.add_argument('window-size=800x841')
• options.add_argument('headless')

Need to extend webtest.py to set the above options and skip Display() for when the environment is OS X.

[hlainchb]

Pull requests are welcome ;)