Refactored AxiosError to a constructor;

Refactored `Cancel` to a constructor, a subclass of the `AxiosError`;
Expose CanceledError class;
Refactored axios error codes;
Added `toFlatObject` util;

test/specs/utils/toFlatObject.js

@@ -0,0 +1,10 @@
+var toFlatObject = require('../../../lib/utils').toFlatObject;
+
+describe('utils::toFlatObject', function () {
+  it('should resolve object proto chain to a flat object representation', function () {
+    var a = {x: 1};
+    var b = Object.create(a, {y: {value: 2}});
+    var c = Object.create(b, {z: {value: 3}});
+    expect(toFlatObject(c)).toEqual({x: 1, y: 2, z: 3});
+  });
+});