Add code for CCITT compression

The class org.apache.pdfbox.pdmodel.graphics.image.CCITTFactory contains code to create a PDImageXObject from an existing TIFF file. Unlike the corresponding JPEGFactory it does not provide any code to create a PDImageXObject from a BufferedImage. The main reason is that pdfbox contains only code to decompress CCITT group 3/4 files, code for compression is missing.

Based on the compression code in Apache Commons Imaging I implemented the missing method and I'm willing to contribute it.
I can provide a patch, but I would need advise whether it's preferable to duplicate code from Apache Commons Imaging or to create a dependency on it.