Initial work on column refactor.
Implement some more methods in DenseColumn.
Add reindex to new Column.
Add force method to Column.
Replace BitSet with custom Mask type.
Remove commented out code.
Cleanup, simplify empty cases, and setNA.

The previous setNA could've forced REALLY large masks for dense columns.
The new setNA will only enlarge the NA mask to, at-most, the size of
values or the NM mask, so it's proportional to the size of the dense
column.
Add equals/hashCode to Mask.
Use sbt-plugin-style mod Id.
Change tixxit/sbt-plugins repo to Ivy style.
Add ColumnBuilders.
Switch dep order so framian dependsOn framianColumn.
Add memoize method to Column.

This method will ensure that values are memoized in the return column.
For empty columns or dense columns, this does nothing. But for evaluated
columns (columns that wrap a function Int => Cell[A]), it will ensure
their evaluation function is evaluated at-most-once (though perhaps
optimistically).
Merge branch 'master' into topic/column-sanity
Add framian-column to aggregate.
Upgrade to sbt-benchmark 0.1.0.

Now with more AutoPlugin.
Fix bug in reindex.

Some NA values were being dropped, which meant the values was undefined
at the tail end.
Add some docs to column.
Add apply method to Column object.
Merge branch 'master' into topic/column-sanity