Renamed Features module to DatabaseFeatures
Stop constant redefinition warnings
Started implementing Dataset features list

The first ones align, but I wrote some specs to be sure
Include DatasetFeatures in Dataset

yeah, that would be helpful
A bunch of things we align with super
We don't support distinct on
We do support GROUP BY ROLLUP but not CUBE
We support INTERSECT ALL and EXCEPT ALL
We do support IS TRUE
A bunch of features where we align with super
Moved supports_* from dataset.rb to features.rb
last of dataset supports_*? methods

610 -> 611 examples
61 -> 62 failures

The new test/failure is that we support rollup, but there's a bug
in it. I've added the bug to the trello board.