Redesign resource manager.

[NEW] Redesigned resource manager. Now it will cache resources which was
loaded.
[MOD] Change the format of saving filename.
Resource manager now can load audio sources.

[NEW] Now resource manager can load audio resources.
Refactor the whole game codes. The version number upgrades to 0.2 now.

[NEW] Add subsystem managers:
- nexus.audio
- nexus.console
- nexus.database
- nexus.graphics
- nexus.input
- nexus.resource
- nexus.scene
[NEW] Add base table:
- nexus.scene.base
- nexus.object.base
- nexus.window.base
[NEW] Add nexus.configures.gameplay and nexus.configures.options
[NEW] Add nexus.system.parameters.saving_slot_size.
[NEW] Now support multi-language texts.
[MOD] Change the saving filename format.
[MOD] Some functions in utility and core are moved to other subsystems.
[MOD] Change all the names "finalizer" to "finalize."
[MOD] Now use Alt + F4 to close the game.