Add group by support for tags.

This commit adds GROUP BY support for tags. Previously only the interval could be grouped by.
Merge branch 'master' of https://github.com/influxdb/influxdb into group-by
Add join support.

This commit adds the ability to join multiple measurements together and calculate aggregate
function expressions.
Merge branch 'master' of https://github.com/influxdb/influxdb into group-by
code review fix.
Code review fixes.
Merge pull request #1247 from influxdb/group-by

Add group by support for tags.