Implement nokia.image-processing localmsg server

This implements the nokia.image-processing localmsg server.

Images are always stored under the /nokiaimageprocessing directory and with an "image-XXX" name. I've decided this because I don't know what Nokia actually does, but looks like the MIDlet I'm testing is making no assumptions on where the file is placed.

The image is always stored as "image/jpeg", because AFAICT the MIDlet I'm testing always uses JPEG images here (when I tried to use a PNG image, the MIDlet said it wasn't supported).

I'll implement a test tomorrow, either in this PR or in a followup.