Allow custom selenium webdriver options via SELENIUM_DRIVER_OPTS

SELENIUM_DRIVER_OPTS can be set to a dictionary that will be passed
inside the MyDriver class to the selenium webdriver. This allows
to use django-selenium combined with custom Firefox profiles or
paths in any of the browsers.

This commit preserves backward compatibility with any existing
setup.
Merge pull request #26 from zeehio/master

Allow custom selenium webdriver options via SELENIUM_DRIVER_OPTS