[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

tests/test_store.py

@@ -0,0 +1,24 @@
+# pylint: disable=C0103,C0111,W0703
+
+import unittest
+
+from bumblebee.store import Store
+
+class TestStore(unittest.TestCase):
+    def setUp(self):
+        self.store = Store()
+        self.anyKey = "some-key"
+        self.anyValue = "some-value"
+        self.unsetKey = "invalid-key"
+
+    def test_set_value(self):
+        self.store.set(self.anyKey, self.anyValue)
+        self.assertEquals(self.store.get(self.anyKey), self.anyValue)
+
+    def test_get_invalid_value(self):
+        result = self.store.get(self.unsetKey)
+        self.assertEquals(result, None)
+
+    def test_get_invalid_with_default_value(self):
+        result = self.store.get(self.unsetKey, self.anyValue)
+        self.assertEquals(result, self.anyValue)