Makefile: test_backprop.o needs config.h; test_backprop doesn't
test_backprop: don't drop the final confab character

It was getting overwritten by the final 0.
test_backprop: use sscanf and urlencoding for metadata

The classify plugin uses sscanf(), but this didn't work when it came
to test_backprop because the metadata values may contain spaces. So I
used a stupid system of field separators which didn't actually work.
Urlencoding the space-containing strings is simpler.
recur-nn: add poorly functioning rprop training

This doesn't work very well and I might pull it out if I can't fix it.
test_backprop: allow negative --rng-seed; use it with --confab-only