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>`

examples/drive_example/src/main.rs

@@ -34,7 +34,7 @@ fn main() {
     let authenticator = Authenticator::new(&secret,
                                            DefaultAuthenticatorDelegate,
                                            client,
-                                           DiskTokenStorage::new(&"token_store.json".to_string())
+                                           DiskTokenStorage::new("token_store.json")
                                                .unwrap(),
                                            Some(FlowType::InstalledInteractive));
     let client = hyper::Client::with_connector(