Set up the storyboard and implemented the LevelViewController.

More details, taken from the message of my second commit in this branch:

"Implemented all methods on the LevelViewController, which are startLevelNumber: and cellPressedAtRow:col:. I removed initializeGridView, initializeGridModel, and rotateClockwisePipeAtRow:col: from the LevelViewController, since I don't think they make sense. I needed methods to get the number of rows and columns from the GridModel, so I made a numRows and a numCols method for it.
I also created a PDCellPressedDelegate protocol, which I expect to be used for both the CellView and GridView. I updated the CellView and GridView's respective delegate properties to use this protocol. I also implemented the GridView's cellPressedAtRow:col: method, since it simply calls the same method on its delegate."