removed all weapons from starting inventory and made weapons purchasable

[ujjman]

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

Weapon	Range	Ammo?	Damage	Cooldown	Time to Kill	Value
Sword	2	no	20-30	500	2000	50
Waraxe	2	no	40-60	1000	2000	50
Staff (Fireball)	long	infinite	10	300	3000	50
Spear (Melee)	4	no	20-30	1000	4000	50
Spear (Throw)	long	yes (self)	100	100 (slot change)	100	50
Crossbow (Arrows)	long	finite	20	200	1000	30 (1 per arrow)

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

[jdrueckert]

Can you please add some information to the PR description detailing why you chose the respective values for the different items? For instance, why does the waraxe cost twice as much as the sword or spear?