added CachedSource mixin

kabuki/sources/cachedsource.py

@@ -0,0 +1,18 @@
+from ..cacher import Cache
+
+
+class CachedSource:
+
+    def __init__(self, size=1):
+        self.cache = cache = Cache(size=size)
+        self.cb = self.add_callback(cache.callback_append)
+
+    def disconnect(self):
+        self.remove_callback(self._cb)
+
+
+
+#TODO: check if source is connected before adding callback
+
+
+