musicus_player: Add basic playback functionality

musicus_player/lib/musicus_player.dart

@@ -1,13 +1,43 @@
 import 'package:flutter/services.dart';
 
+/// A simple music player.
+/// 
+/// Give it an URI using [setUri] and it will start playing.
 class MusicusPlayer {
+  /// Called, when the player reaches the end of the audio file.
+  final void Function() onComplete;
+
   final _channel = MethodChannel('de.johrpan.musicus_player/platform');
 
-  MusicusPlayer() {
+  /// Create a new player.
+  /// 
+  /// This will do nothing, until [setUri] was called. If the player reaches
+  /// the end of the current audio file, [onComplete] will be called.
+  MusicusPlayer({this.onComplete}) {
     _channel.setMethodCallHandler(_handleMethodCall);
   }
 
-  Future<void> _handleMethodCall(MethodCall call) {
-    // TODO: Implement.
+  Future<void> _handleMethodCall(MethodCall call) async {
+    if (call.method == 'onComplete' && onComplete != null) {
+      onComplete();
+    }
+  }
+
+  /// Set URI of the audio file to play.
+  /// 
+  /// If the player will always stop doing, what it did before, and start
+  /// playing from the provided URI if possible.
+  Future<void> setUri(String uri) async {
+    await _channel.invokeMethod('setUri', {'uri': uri});
+  }
+
+  /// Play from the current URI and resume playback if previously paused.
+  Future<void> play() async {
+    await _channel.invokeMethod('play');
+  }
+
+  /// Pause playback.
+  Future<void> pause() async {
+    await _channel.invokeMethod('pause');
   }
 }