Implemented SeekTo method.

[JuanuMusic]

I needed to be able to navigate to a specific location in milliseconds, so I just went ahead and implemented it.

The VideoPlayer class now has SeekTo and CanSeekTo methods. Both receive a specific location in milliseconds as a parameter.

I know Seek works with seconds, but it's for convenience. In the case of SeekTo, we need to be able to specify a specific time.

Let me know what you think!

[JuanuMusic]

This issue (https://github.com/adamfisher/Xamarin.Forms.VideoPlayer/issues/174) could be solved using this method.