Allow DISTINCT function to operate on tags

I would like to have following feature:

Since the new release 0.9.3 tags are resulting as own columns if you use SELECT \* FROM measurement

Currently, it's not possible to use commands on this columns. An example:

```
SELECT * FROM measurements
```

returns:

```
time tagA tagB value
xxx M N 0.3
xxx M O 0.4
xxx P R 0.2
```

I want to do a query like:

```
SELECT count(distinct(tagA)) FROM measurements
```

The result is

```
2 (M+P)
```

Anyone else need this feature?