modularize src

+ merge main.py and manage.py
+ move start logic to swingmusic/__main__.py
+ add a run.py on the project root

swingmusic/utils/decorators.py

@@ -0,0 +1,11 @@
+def coroutine(func):
+    """
+    Decorator: primes `func` by advancing to first `yield`
+    """
+
+    def start(*args, **kwargs):
+        cr = func(*args, **kwargs)
+        next(cr)
+        return cr
+
+    return start