Added the ability to disable hooks

[Ayuto]

Generic fix for issue #236

Test code:

from memory.hooks import hooks_disabled
from entities.hooks import EntityPreHook, EntityCondition
from events import Event
from players.entity import Player

@EntityPreHook(EntityCondition.is_human_player, 'on_take_damage')
def pre_take_damage(args):
    print('take damage', args)

@Event('player_say')
def on_player_say(event):
    player = Player.from_userid(event['userid'])
    target = player.view_player
    if target is None:
        print('not a player or no target')
        return

    with hooks_disabled(event['text'] == 'yes'):
        target.take_damage(5)

When looking at a player and typing yes, the hook shouldn't get called. In all other cases the hook gets called.