Export figures feature (#70)

This adds a `make export_figures` command which exports all the flow
charts into nicely looking pngs. The command requires that you have
docker running locally.

book/figures/export_figures.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+for i in *.pdf
+do
+  docker run -v $(pwd):/imgs dpokidov/imagemagick -density 900 -trim /imgs/$i -quality 100 /imgs/$i.png
+  rename 's/pdf.png/png/' *.png
+  rename 's/fig-/figure-/' *.png
+done
+