Removed singletons except Engine class & Engine no more relies on nini and ini files 

Configuration Changes
* Moved ini-file based config-classes from Engine to Sample Game project. Engine is now free of both Nini reference and the ini file itself. The actual game show now do what ever it wants (ie . reading from the ini file) supply parameters to EngineConfig.
* Moved EngineConfiguration class to Core/Config and renamed as EngineConfig. Also added AudioConfig and GraphicsConfig classes.
* Renamed Config.cs as SettingsReader.cs.
* AssetManager is no more singleton and now instead referenced by Game.Services.GetService() request (issue #43).
* ChunkStorage and InputManager are no more singletons now.
* Rasterizer is no more a singleton.
* Now nothing is singleton except the Engine class itself based on the discussion here: https://github.com/raistlinthewiz/voxeliq/issues/43
* closes #43.

Singletons
* The Engine ctor() will now throw an exception when additional instances created (Discussed in issue #43).
* SettingsReaders are no more singletons.

Misc
* Renamed Game.cs in Game project to SampleGame as it was conflicting with XNA.Game.
* Removed GeneratedblockStorageTest.cs as it was reduntant.
* Tests will now check if Engine is already initiated and dispose it first on test startup.
* Also added Test teardown methods.