Adding default value for selectable in column options, optimizing canCellBeSelected.
Merge pull request #540 from dariusk/master

Default selectable column option, minor optimization of canCellBeSelected
Added support for predefined group values to DataView.

New grouping info properties:
- predefinedValues - An array containing the grouping values for which
groups should be included even if they are empty.
- aggregateEmpty - Whether aggreggators should be run on empty groups.
Merge remote-tracking branch 'mleibman/master'
Merge branch 'master' into 2.0-frozenRowsAndColumns