Upload SVG images and dot/circo source files

* Prefixed my old image names with en- for future translation

* Uploaded source .dot files for all previous images

* Changed my old images from PNG to SVG. Also tweaked old images to look
  better at 600px wide

* Updated en/developer-guide.md to use new image locations

* Uploaded five new images for forthcoming payment processing section.
  One image generated using circo, so README instructions updated.
Adding PNG versions of all SVG files from img/dev

For browsers which don't support SVG
Merge pull request #37 from harding/images

Upload SVG images and dot/circo source files