Implement EntityCollection.extend() method for Issue #27

Added extend() method to EntityCollection that accepts any iterable
of Entity objects and adds them all to the collection. The method:
- Accepts lists, tuples, generators, or any iterable
- Validates all items are Entity objects
- Sets the grid association for each added entity
- Properly handles errors and empty iterables

closes #27

src/UIGrid.h

@@ -77,6 +77,7 @@ class UIEntityCollection {
 public:
     static PySequenceMethods sqmethods;
     static PyObject* append(PyUIEntityCollectionObject* self, PyObject* o);
+    static PyObject* extend(PyUIEntityCollectionObject* self, PyObject* o);
     static PyObject* remove(PyUIEntityCollectionObject* self, PyObject* o);
     static PyMethodDef methods[];
     static PyObject* repr(PyUIEntityCollectionObject* self);