Clean up stage loading screen

- Perfectly center the loading box (responsive to screen size)
- Remove the green background
- Make sure the percentage never goes beyond 100%

Issues: https://github.com/AUTUpStageTeam/UpStage2013/issues/52, https://github.com/AUTUpStageTeam/UpStage2013/issues/12, https://github.com/AUTUpStageTeam/UpStage2013/issues/20