Feature/screenshot

Hi, i've added this two things:
  
   * __screenshot__ method in AdbSdkLib::Device that wraps __getScreenshot()__
   * __AdbSdkLib::RawImage__ as a wrapper of __com.android.ddmlib.RawImage__ with some improvements

Now it's possible to make screenshots getting the frame buffer as a RawImage, [here](https://gist.github.com/encrypt94/9de6a45f728748b63872) is an example of how to use new features.