Added CefSharp.Offscreen, CefSharp.Offscreen.Example projects.

Please run CefSharp.Offscreen.Example project to test this out (it is a Console application which takes a screenshot of the Google homepage).

I have ensured everything works in Debug/Release and x86/x64 mode, however I am unsure about the nuget  packages as I don't know how to build them (and was hoping someone else could tell me if it works :wink:).

Addresses issues #577, #518 (sort of; about printing, but screenshots may be acceptable), #462.