Added RowHandler to handle row completion/deletion
Merge branch 'master' of https://github.com/bbrouse/shapesmith into bbrouse_issue_13
Row deletion on completion is close to being done.  Still some bugs to work out.
Merge branch 'master' of https://github.com/bbrouse/shapesmith into bbrouse_issue_13

Conflicts:
	shapesmith/Assets/Prefabs/I-Tetromino.prefab
	shapesmith/Assets/Prefabs/Z-Tetromino.prefab
	shapesmith/Assets/Scripts/PlacementManager.cs
	shapesmith/Assets/_Scenes/main.unity
	shapesmith/shapesmith.userprefs
Row completion now working (cubes get destroyed on completion).  Improved performace of gravity.
Updated tetromino prefabs for row completion/gravity changes
Added comments to document row handling code
Merge pull request #26 from bbrouse/bbrouse_issue_13

Row completion and gravity performance