add support for floating drawings

- added need elements and test for them

src/file/drawing/floating/position-offset.spec.ts

@@ -0,0 +1,14 @@
+import { assert } from "chai";
+
+import { PositionOffset } from "./position-offset";
+import { Utility } from "../../../tests/utility";
+
+describe("PositionOffset", () => {
+    describe("#constructor()", () => {
+        it("should create a element with correct root key", () => {
+            const newJson = Utility.jsonify(new PositionOffset(50));
+            assert.equal(newJson.rootKey, "wp:posOffset");
+            assert.equal(newJson.root[0], 50);
+        });
+    });
+});