Refactor the codes.

[NEW] Now nexus.object.* have create and delete methods.
[NEW] Now nexus.scene.* have create and delete methods.
[NEW] Now nexus.window.* have create and delete methods.
[ENH] All the scripts reference to local nexus table, not global looking
up.
[ENH] Improved code qualities.
[FIX] Fixed the problems of nexus.scene.loading.
Add nexus.base.rectangle.

[NEW] Add nexus.base.rectangle.
New nexus.base.viewport.

[NEW] Add nexus.base.viewport.
[NEW] Add table.indexOf().
[NEW] Add table.removeValue().
[NEW] Now can use viewport to manage drawables.
[MOD] Now nexus.window.* use internal viewport to render.
[MOD] The nexus.scene.stage use canvas to draw grid for developing.
[MOD] The commands of nexus.scene.title all use nexus.core.scene.goto().