Add configuration to change background image of PJIRC

It should be possible to change the background image por Pjirc applet directly from Drupal Pjirc settings.
Now it is only possible to do by overwriting pjirc/pjirc/background.gif file via FTP/shell.

I'm not sure how to implement this. Dealing with uploaded files is a bit of a hassle, and because this would probably be an user-uploaded image, it should not reside in the module dir. Also that won't be good for updates as long as the file may be deleted or overwrited.

So this has two parts:
- How and where upload the image
- How to select this image from Pjirc configuration.

This may be converted into one only part if we only have one image for background, overwriting it (or deleting the previous one) every time we upload a new one. Then Pjirc will use this uploaded image if it exists or use the default otherwise.

The latter is probably what I will implement.

I would also add an option to not include any background image.