svg external bitmap

This patch enables saving bitmaps in svg as external files, so as to reduce output size and rendering costs, and allow easier post-processing of bitmaps.

Currently only DCT images can be saved as jpg, other images are still embedded in svg.

@coolwanglu Average size of images in a PDF is usually a few KB or more, so the effect of adding a few bytes to the file name would be small. I note that in a cairo generated SVG, images are always defined in `<defs>` and referenced indirectly, so a url to image would occurs at most once per page. Images that occur on many pages would be relatively a small portion.

If we use hex numbers, each time I want to search the obj in the PDF, I have to convert it to decimal in a calculator first, it is a pain (have been encountered it with bgXXX.svg).

So at least, give user an option to use decimal instead of hex. 

Shorter prefix is ok to me. However, `o(\d+).jpg` doesn't work for hex number.

I want to avoid creation of additional temp object, however I just remember [Return value optimization](http://en.wikipedia.org/wiki/Return_value_optimization), so may be it's OK to just create and return a string.