Automated installation of Opera 12 extensions through operaProfile.addExtension(File)

Fixes #60

The method has the signature `addExtension(File)` to be consistent with the FirefoxDriver.

Besides implementing the feature, I have also updated the code of the webserver, because it prevented me from running tests. Apparently, the webbit API changed 4 months ago, without remaining backwards-compatible.

I've included a test that passes, so installation of an Opera extension succeeds.
Not included in the test suit, but confirmed to work is the use of the Remote WebDriver with a custom extension.

Details:

- Local OperaDriver successfully tested on ArchLinux + Opera 12.15
- Remote WebDriver tested through Selenium Grid with Windows XP + Opera 12.00
- Using Selenium 2.33.0