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/transform.spec.js

@@ -1,3 +1,5 @@
+var AxiosError = require("../../lib/core/AxiosError");
+
 describe('transform', function () {
   beforeEach(function () {
     jasmine.Ajax.install();
@@ -64,6 +66,7 @@ describe('transform', function () {
         setTimeout(function () {
           expect(thrown).toBeTruthy();
           expect(thrown.name).toContain('SyntaxError');
+          expect(thrown.code).toEqual(AxiosError.ERR_BAD_RESPONSE);
           done();
         }, 100);
       });