Use the power of the `AsRef` trait to take generic parameters for
several API functions. This makes the API more ergonomic because the
callers may pass in static `str` slices or references to owned `String`s
or even more exotic things like a `Cow`, all based on their particular
situation.
Update the tests and examples to use the most natural types they have
available.
Fixes#77. No existing code should break, as `&String` implements
`AsRef<str>` and `AsRef<Path>`
I mainly resolved some circular dependencies that had crept in, and
moved code around. I renamed helper.rs because that was not really an
appropriate name anymore, and moved the delegate code into a new module.
DiskTokenStorage is a TokenStorage that stores its tokens in a JSON file
on disk. That file can be read in later, and the tokens in it reused.
(The idea for a cache file is from here:
https://developers.google.com/drive/v3/web/quickstart/go)
