Implement BTCChina Data API v1.3.1: Added two more return values "vwap" and "prev_close" for ticker API.

[Data API v1.3.1](http://btcchina.org/api-market-data-documentation-en#data_api_v131): 2014-08-21 Data API v1.3.1: Added two more return values “vwap” and “prev_close” for ticker API.