Add regression test for grouping by 3 columns.

Related to issue #215.
Merge pull request #216 from slick/tmp/group3-bug-2.0

Add regression test for grouping by 3 columns.
Add hashCode() implementation to ProductValue.

Also optimize GroupBy implementation in QueryInterpreter to avoid
unnecessary copying of data.

Fixes issue #215 on MemoryDriver (regression test was failing).
Merge pull request #221 from slick/tmp/productvalue-hashcode

Add hashCode() implementation to ProductValue.
Merge branch 'master' into feature/DBDefault
got latest updates from master
changed Default in ColumnOptions to take a DefaultColumn[T] instead
this will prevent erroneous column[T] being passed in and compiling