Column Widget

Adds a column widget to be nested within a column of a row widget. This is the first iteration that allows for a max of four widgets in a column. The next iteration will (hopefully) allow for an unlimited number of widgets within the column. We wanted to have something shipped for Wednesday's demo, so we opted to go with this approach.

https://www.pivotaltracker.com/n/projects/642513/stories/72127722
can row-content be column-content?