Flatten nested ProductNodes in GroupBy keys.

Databases without tuple syntax obviously don't support tuples in
"group by" clauses, so we have to flatten them. We can safely do this
in the convertToComprehensions phase. It should not affect semantics
for databases that would support tuples in this case.

Test case in AggregateTest.testGroup3. Fixes issue #207.
Merge pull request #223 from slick/tmp/nested-group-key

Flatten nested ProductNodes in GroupBy keys.