Created ResourceManager class which will be used to easily handle mappings between objects and their resources (e.g. sprites or textures).
Added ResourceIdentifier interface which all classes that want a resource associated with it need to implement.
Added more javadoc.
Added simple test case for an Enemy using the ResourceManager. Also added EnemyType to Enemy constructors so that it can be retrieved mor easily.
Fixed tests.
Added new picture for enemies to use.
Fixed bug where Enemy objects would share texture despite having different textures set.
Merge branch 'resourceManager' into development