GUI

ADDED buttons on bottom. The first two currently do the same thing. They
change modes to place towers. Second one is for walls but dont know
where wall placement is done.

CHANGES to Buy_Shoot_Modes to make shootMode and buyMode public.
Health Bar

Added basic health bar. Will add background/style to it later.
GUI-fixed
added enemy health to flying enemies
added cost to turrets/walls, GUI for resource
quick changes, balancing
tower ammo destroyed when it hits enemy.

Before it would get destroyed if it hits any collider in the scene so if
it is next to a turret since it has a wide radius as a collider the
tower ammo gets destroyed when fired in the vicinity of a tower
quick balancing changes