Two simple fixes in FluxJpeg

Hi - 

I've forked this project and committed a few fixes that I needed for our customer project. I originally forked to googlecode project and noticed that the developers using the library had the same difficulties we did. 

The first problem we encountered was that the density properties can be updated but never make it into the output file when re-encoding the image. JpegEncoder never touches the JFIF or EXIF headers that contain this info.

The second problem was that hierarchical Jpeg's weren't supported. Luckily the library already knew about this problem and threw an exception. 

Both of these issues have been reported in the original google repository and I was surprised that they weren't fixed 5 years later. After changing the code for our customer project I found out about the GitHub repository and decided to fork this one. 

This pull request revolves around these two problems. I had to create GitHub account for this.