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.

[If Saïvann doesn't beat me to it, I will merge this cosmetic-only pull request in a few hours when I finish proofreading the forthcoming payment processing section.]