Added new references for the images corresponding to the various exit types that can occupy a Cell.
Factored the population of Cells within the level out into its own private method.  Also added complete Javadoc for the class.
Changed some method names.
Rejigged some of the tests to work with refactored code.
Added tests for the Cell class.
Entity is no longer an enum, rather it is now an Interface.  Introduced a new class for exits called Connection, with an enum ConnectionType that determines what kind of exit it is.  This new setup is more in line with OO principles.  Borders and images are now added polymorphically rather than a per class basis.  Rooms will implement the marker interface Connectible which indicates to the client that an exit can be attached to it.
Added JavaDoc.
Reworked the code so that it will work with the newly refactored Model layer.
Added JavaDoc and simplified the means by which Rooms contained within the Level are added/removed and accessed.
Factored the code for deleting an Entity from a Cell out into its own class.
Added JavaDoc and comments.  Cleaned up a lot of the code for better clarity.  Made changes so that the code now works with the refactored Model layer.