Try for the Select Distinct.

Hi.  
At first, I did read https://github.com/slick/slick/issues/96
This work is for the Select Distinct. 
In the Slick, in order to use the select distinct, below syntax is used. 
```
qA.groupBy(t=>t).map(_._1)
```
And, result sql is like below. 
```
select x1.colA, x1.colB from (...) x1 group by x1.colA, x1.colB
```
In this work, below syntax is possible, also. 
```
qA.distinct
```
And, result sql is like below. 
```
select distinct x1.colA, x1.colB from (...) x1
```
This doesn't introduce any Node classes. Instead, add a one flag into Comprehension. 
And, the `.distinct` method is just a macro for `groupBy(t=>t).map(_._1)`.  
Thanks.