Code tidyup, bug fix, work on touch support

FlashSprite function now available to all game objects that inherit
CustomBehaviour over MonoBehaviour

Manager classes now accessible globally
- Use their static member "Instance"
- Updated code to use this member

Separated player input logic from the main player class
- Started work on support for touch devices

Player
- Removed redundant code
- Clarified StartFireCooldown timer
- Fixed bug where player was not getting hit by the meteor upon contact
consistently
- Clarified CanShootBullet as a property to match its intended use

Bullet
- Updated to use built-in Unity functions OnDisable