[core] Add "Store" interface

Add an interface that allows arbitrary objects to store/retrieve
arbitrary key/value pairs. This will be used for different purposes in
the future:

* Config class(es) can store user-defined parameters for modules
* Widgets can store state
* ???

see #23

bumblebee/store.py

@@ -0,0 +1,18 @@
+"""Store interface
+
+Allows arbitrary classes to offer a simple get/set
+store interface by deriving from the Store class in
+this module
+"""
+
+class Store(object):
+    def __init__(self):
+        self._data = {}
+
+    def set(self, key, value):
+        self._data[key] = value
+
+    def get(self, key, default=None):
+        return self._data.get(key, default)
+
+# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4