Add support for Silverlight

Add support for Silverlight exploitation. [SeeRM #8705]

I decided that the check is probably best just returning true or false, because you can only detect the version with ActiveX, and that's an IE thing. To support multiple browsers, the best you can do is detect if the plugin is installed or not. So when setting your browser exploitable requirements, make sure you do :silverlight => "true" or "false". Several files are updated, including the recent MS13-022 exploit.

I also tested the check against these setups:

Windows: IE 8/9, latest Safari, Mozilla Firefox, Opera, Google Chrome.
OS X: Safari, Firefox, Google Chrome.