Teh/linesearch

I cherry-picked improvements on the `teh/constrained` branch (those that didn't involve the speculative interior point method), because I didn't want a merge headache. Then I got rid of Options. Should fix #71, #75. May also address #79, but I haven't looked yet.
Oh, crap. I was just working my way through the README, and I thought there wasn't a `rosenbrock_and_gradient`. I just didn't scan far enough down. I'll change this back.