Add ffprobe information, passing tests in /Tests/ directory

Hello @kmisiunas,

Thank you for the great work on this package, and your information posted on [SOF](http://mathematica.stackexchange.com/questions/18335/how-to-efficiently-import-videos)

I would like to use ffmpeg not directly for import speed, but to be able to access videos with codecs not supported by Mathematica. For that, one needs to implement import of things like "FrameRate", "ImageSize", etc. I did this by parsing the log output of `ffprobe`, it isn't the most elegant implementation, but it is passing tests and it agrees with the output of Mathematica's native `Import[ file, "Duration"]`, etc. (see new test notebook)

I didn't replace the `Import[]` statements used in your main code yet, but I would like to replace them with `FFImport[]`. Please kindly let me know what you think!

Sincerely,
Kenta Takagaki