imp: accept any string-like parameter

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>`

src/authenticator.rs

@@ -238,8 +238,7 @@ where
                                     }
                                 }
                                 RefreshResult::RefreshError(ref err_str, ref err_description) => {
-                                    self.delegate
-                                        .token_refresh_failed(&err_str, &err_description);
+                                    self.delegate.token_refresh_failed(err_str, err_description);
                                     let storage_err =
                                         match self.storage.set(scope_key, &scopes, None) {
                                             Ok(_) => String::new(),