Look for chromedriver on the system PATH too

Note that the PATH when running in your IDE may not match the PATH in your shell.  I had to put a symlink for `chromedriver` into `/usr/local/bin`, because the PATH was only: `/usr/local/bin:/usr/bin:/bin`
DISPLAY is global env variable right? This webdriver.display property is to enable webdriver to run in its own display such as xnest or Xephyr.