Adding SELENIUM_URI to allow host, port and path to be set at once.

Hi Ed,

I've discovered selenium server run from the jar is different to chromedriver2 service on my mac. I need to add the path /wd/hub when using selenium server on my linux jenkins build slave. I've added the env variable SELENIUM_URI which allows you to set protocol, host, port and path at once. If SELENIUM_URI is not defined it will fallback to SELENIUM_HOST and SELENIUM_PORT usage. This change should be backwards compatible and not affect existing users.

Can you accept this change?

All the best,

Oisin