Added Ammo buying system

You can now use the weapons tab to purchase ammo in the fighting round.
Although your guns now stay in your inventory from round to round and
death to death. Your Ammo will not, so although you don't need to worry
about buying weapons over and over, you should only buy the ammo that
you need. You cannot buy ammo outside of the fighting phase.

The next update will focus on clarifying how the gamemode works and then
changing some things design wise.