Add simple headless testing support with PhantomJS

added phantomjs option in start_webdriver_client
Merge pull request #123 from kevin-miles/patch-1

Add simple headless testing support with PhantomJS
Load selenium defaults from environment variables

The following options will attempt to load values from the corresponding
environment variables:

 --browsername, SELENIUM_BROWSER
 --browserver, SELENIUM_VERSION
 --platform, SELENIUM_PLATFORM
Prefix help string for webdriver-specific options
Adding Appium support for Saucelabs