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.
Merge pull request #168 from craigcitro/oob

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