Fixes #248
This PR removes all weapons from starting inventory. It makes weapons purchaseable and attempts to balance them a bit better.
We do have different dimensions along which we can assess the (combat) value of a weapon:

• range
• damage
• ammo requirements
• cooldown

Time to kill is ceil(playerMaxHealth / damage) * cooldown with playerMaxHealth = 100.
Starting money for players is 100.

Melee weapons - sword, axe (, staff, spear)

• Con: need to get close to enemy
• Pro: acceptable to high damage

Ranged, ammo-less weapons - staff (fireball)

• Con: low damage, hard to hit opponent
• Pro: endless ammo, acceptable cooldown, long range

Ranged, ammo-requiring weapons - Crossbow (arrows)

• Con: requires ammo, hard to hit opponent
• Pro: low cool down, acceptable damage, long range

Ranged, pick-up weapons - Spear (throw)

• Con: needs to be picked up, hard to hit opponent
• Pro: very high damage, long range