Default to the "oob" flow if httpuv isn't installed.

This modifies the logic in oauth token fetching to default to using the `oob`
flow (i.e. provide a URL and ask the user to copy-paste a code) whenever
`httpuv` isn't available to use locally.

PTAL @hadley @deflaux
done -- i'm now in the dubious position of having a condition like `(!a && !b)`, which i generally try to avoid -- but in this case i think it reads more clearly than `!(a || b)`. feel free to disagree. `:)`

fixed the `=` ... my days spent with python are showing.