Fix multiple low priority issues

closes #12, closes #80, closes #95, closes #96, closes #99

- Issue #12: Set tp_new to NULL for GridPoint and GridPointState to prevent instantiation from Python
- Issue #80: Renamed Caption.size to Caption.font_size for semantic clarity
- Issue #95: Fixed UICollection repr to show actual derived types instead of generic UIDrawable
- Issue #96: Added extend() method to UICollection for API consistency with UIEntityCollection
- Issue #99: Exposed read-only properties for Texture (sprite_width, sprite_height, sheet_width, sheet_height, sprite_count, source) and Font (family, source)

All issues have corresponding tests that verify the fixes work correctly.

src/UICollection.h

@@ -28,6 +28,7 @@ public:
 	static PyObject* subscript(PyUICollectionObject* self, PyObject* key);
 	static int ass_subscript(PyUICollectionObject* self, PyObject* key, PyObject* value);
 	static PyObject* append(PyUICollectionObject* self, PyObject* o);
+	static PyObject* extend(PyUICollectionObject* self, PyObject* iterable);
 	static PyObject* remove(PyUICollectionObject* self, PyObject* o);
 	static PyObject* index_method(PyUICollectionObject* self, PyObject* value);
 	static PyObject* count(PyUICollectionObject* self, PyObject* value);