add failing test for #2
find simpler input that tirggers the failure
fix #2 by special-casing ^

... since it runs before we pull unary -, so it looks like a binary minus is unary to the unary minus puller. ugh.
update readme with now-working output for x^2-y^2+time*3000
Merge pull request #3 from uniphil/bug/parser

fix #2 regression