Continued NetworkObserver-related implementation
Merge branch 'Canfield-Staging' into Canfield
Merge pull request #321 from BU-CS673/Canfield-Staging-2

Canfield-Staging-2 -> Canfield
NetServerTest & NetClientTest implement NetworkObserver
Continued implementing NetworkObserver functionality
Player names are now sent across the network on start
Initial game lobby work
Added onMessageReceived to NetworkObserver
Removed AppState work - not needed for now
UiScreen work
Updates to NetworkLobby
Genericized DrawLayer

It no longer has a direct name relation to the Entity inheritance
hierarchy, to emphasize that the DrawLayer and the Entity inheritance
hierarchy are not related.
Removed 2 redundant dispose tests

These are already tested elsewhere.
Many more network command unit tests
Unit test fixes
Added 2 more NetworkSystem unit tests
Changed 3 classes to abstract

Because they only have static methods (Coordinates did already have a
private constructor, which accomplishes the same thing)
Comment typo fixes; Removed outdated TODOs
Reduced TEXTURE_FILE visibility to private in sprites
Renamed DrawLayer.BOTTOM -> DrawLayer.FIRST