Added support for file() command

This adds support for file uploads via the yet-to-be-documented file() command. The resulting remote path location may then be used as the value() of file input fields.

References:
* https://github.com/aivaturi/Selenium-Remote-Driver/commit/9d4ea565f2b3d48ab91bfe6f880f7881c6dcf06e (where the logic was mostly borrowed from)
* http://sauceio.com/index.php/2012/03/selenium-tips-uploading-files-in-remote-webdriver/