nexpect now requires node >= 0.8

It used to claim to support node >= 0.2. I don't know if it supported
0.2, but it definitely didn't support node 0.10. Currently, tests pass
with 0.8 and 0.10, the last two stable releases.
Fix wait tests by having test fixture exit

I don't know why prompt-and-respond doesn't exit when run as a child
process, but does when run at the command line, and I don't care right
now, the tests now pass.
Return exit code, or signal if it was killed
Export API documentation from code to README
Update vows and prompt dependencies

Versions depended on are years old, and both have improved a lot,
vows in particular supports xunit output.