values allowed for time function in group by clause

Based on ParseDuration docs [http://golang.org/pkg/time/#ParseDuration] which is being called  in GetGroupByTime [https://github.com/influxdb/influxdb/blob/master/src/parser/parser.go#L89] at [https://github.com/influxdb/influxdb/blob/master/src/parser/parser.go#L99].