Use keyring

[jonthegeek]

As a package developer, in order to encourage best practices, I would like to provide keyring functionality.

For auth, create a function along the lines of get_default_key(), which reads from {keyring} if installed, else an envvar. Also create a setter function for the API. Most of this functionality should be bundled in nectar, but still give the API-wrapping package a version with named keys, etc.