Add an example to fetch a Spotify API token (#512)

examples/get_token.rs

@@ -0,0 +1,37 @@
+use std::env;
+use tokio_core::reactor::Core;
+
+use librespot::core::authentication::Credentials;
+use librespot::core::config::SessionConfig;
+use librespot::core::keymaster;
+use librespot::core::session::Session;
+
+const SCOPES: &str =
+    "streaming,user-read-playback-state,user-modify-playback-state,user-read-currently-playing";
+
+fn main() {
+    let mut core = Core::new().unwrap();
+    let handle = core.handle();
+
+    let session_config = SessionConfig::default();
+
+    let args: Vec<_> = env::args().collect();
+    if args.len() != 4 {
+        println!("Usage: {} USERNAME PASSWORD CLIENT_ID", args[0]);
+    }
+    let username = args[1].to_owned();
+    let password = args[2].to_owned();
+    let client_id = &args[3];
+
+    println!("Connecting..");
+    let credentials = Credentials::with_password(username, password);
+    let session = core
+        .run(Session::connect(session_config, credentials, None, handle))
+        .unwrap();
+
+    println!(
+        "Token: {:#?}",
+        core.run(keymaster::get_token(&session, &client_id, SCOPES))
+            .unwrap()
+    );
+}