Added ConfigureAwait(false) to avoid UI thread blocking

Reference:
http://social.msdn.microsoft.com/Forums/en-US/269172a3-adb9-4b5e-9ac1-8b67ff920177/async-issuewhy-blocking-the-ui-thread?forum=async#48b596cc-5eca-4d0e-8743-10d59b64e2ce