Allow custom selenium webdriver options via SELENIUM_DRIVER_OPTS

Please consider merging the following patch to django-selenium.
It addresses in a general way issue #16, offering the possibility of passing
custom arguments to the selenium webdriver.

It can be used to pass custom paths for firefox, chromedriver, etc and also
to pass custom options for each webdriver, such as a custom firefox profile.

I have added the respective documentation, to make adoption of the patch
as easy as possible.

Any comments will be very much appreciated. Thank you for your time.