Add image move support

Hi!

I was viewing [this](http://www.reddit.com/r/programming/comments/1zsnde/thinking_about_quickly_writing_an_http_server/) earlier today when I noticed how bad RES is at displaying pictures when you make the image larger than your screen. In the particular picture I linked you will probably only be able to read the top left part of the picture, because the rest of it disappears in the void that is the outside of your screen.

I added the ability to move images by dragging (as you do when resizing) and holding down shift (actually only holding shift at the start of the drag is enough).

Please be ge[m]tle, first pull request.

Tested on Chrome Version 33.0.1750.146 on Ubuntu 13.10 64 bit.