function addImage for a PhotoSwipe instance added (Issue #243)

Is this code possible? I'm not really in the code of the whole project and just hacked this in the source and for my project it is working. There need to be some checks on the obj passed but I first wanted to have some comments on the general idea of the code.

I think an option to add an image dynamically is very useful. I needed to add images from an ajax call and didn't want to reinitialize the whole PhotoSwipe instance.

I'm new to contributing on GitHub, so if I'm doing something wrong here GitHub specific, please tell me :)