Support timezone in ISO8601Long date format with date ending with 'Z' which mean UTC timezone

Recently, I found issue with jqGrid show date from mongo incorrectly since mongodb keep date as date time in UTC.

Therefore, I change the the parstDate to parse ISO8601Long date correctly when the date end with 'Z' which mean UTC time.

Could you please review this change and merge to jqGrid?

reference: http://www.w3.org/TR/NOTE-datetime