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;

lib/defaults.js

@@ -2,7 +2,7 @@
 
 var utils = require('./utils');
 var normalizeHeaderName = require('./helpers/normalizeHeaderName');
-var enhanceError = require('./core/enhanceError');
+var AxiosError = require('./core/AxiosError');
 
 var DEFAULT_CONTENT_TYPE = {
   'Content-Type': 'application/x-www-form-urlencoded'
@@ -90,7 +90,7 @@ var defaults = {
       } catch (e) {
         if (strictJSONParsing) {
           if (e.name === 'SyntaxError') {
-            throw enhanceError(e, this, 'E_JSON_PARSE');
+            throw AxiosError.from(e, AxiosError.ERR_BAD_RESPONSE, this, null, this.response);
           }
           throw e;
         }