Added entity instances caching.

[jordanbriere]

I was curious about the feasibility of the caching @Ayuto suggested into #284 so I played a bit with the idea and ended with this implementation. Entity instances are cached by default, unless caching=False is passed to the constructor. So, if users want their private instances for whatever reason, they can do so. Ran the following code to time the difference and the results are considerable:

from time import time
from players.entity import Player

class Cached(Player):
    pass

t = time()
for i in range(1000000):
    Cached(1).name
print('Cached:', time() - t)

class NotCached(Player):
    pass

t = time()
for i in range(1000000):
    NotCached(1, caching=False).name
print('NotCached:', time() - t)

Results:

Cached: 0.9825160503387451
NotCached: 5.010047912597656

And here is the code I used to ensure instances were not broken by this implementation:

from entities.entity import Entity
from players.entity import Player
from weapons.entity import Weapon

ent = Entity(0)
assert ent is Entity(0)
ent.index
ent.origin
ent.properties

player = Player(1)
assert player is Player(1)
player.playerinfo
player.name
player.speed

weapon = Weapon._obj(player.give_named_item('weapon_m4a1'))
assert weapon is Weapon(weapon.index)
weapon.clip
weapon.ammo
weapon.index
weapon.next_attack
weapon.remove()

class MyPlayer(Player):
    @property
    def test(self):
        pass

my_player = MyPlayer(player.index)
assert my_player is not player
assert my_player is not MyPlayer(player.index, caching=False)
my_player.name
my_player.test

[Ayuto]

Awesome! Looks good :)