Add column alias support

[iaddict]

It would be nice if one could give aggregated columns alias names.

The select clause thus should allow the following syntax:

select max(temp) as max_temp, min(temp) as min_temp from sensor_data group by time(1h);
select max(temp) max_temp, min(temp) min_temp from sensor_data group by time(1h);

[pauldix]

This is a duplicate of #69, should be in 0.6.0 release.