Adds first steps of Flappy.

Lots of code changes to accomodate the new general:
  * Implementes Arc area, that also doubles as a path.
  * Generals are now the responsible of placing their own minions at
    the start of the battle.
  * Moving entities now return True or False when move() is invoked.
  * Entities can now follow a Path. Pathing effect implemented.
  * New skill that adds a path to an entity.
  * Generalized BigEntities so they can be larger (but still squared).
  * Adds Mechanics faction.
  * Some a bit more.
Converts the Slingshot into a BigMinion.

Remember when I said that the difference in order of the side, x and
y parameters in Entity and Minion could cause confussion? I just
proved it true.
New graphics for Flappy's slingshot.

The slingshot is drawn when the cooldown is ready and undrawn when
it's not. You need to have the slingshot drawn and the Gobmerang on
it to be able to launch it.
Adds new Flappy skill: drop a boulder.

The boulder takes some time to land and then gradually loses speed
doing less damage on the first impact (and getting stopped there).
If it looks like the boulder misses some enemies it's because it's
bouncing.
Boulder doesn't stop anymore on a hit.

It will continue its path jumping and crushing all over the place.
Also now it seems bigger before reaching the floor and smaller when
it's on the ground.
All entities show some basic contextual info.

Closes #22.
Adds Flappy's inners when the slingshot is dead.
Adds a new skill for Flappy: burn them from above!

Statuses get a new parameter, name. There can't be two statuses with
the same name in the same entity, if you try to apply a new one, it
will be disregarded but the duration of the original could be
extended.
Merge branch 'master' into mechanics

Conflicts:
	entity.py
	minion.py

All the stuff related to issue #20.
Adds exploding machine skill to Flappy.

Implements new CustomArea to make it work.
Makes arc return an empty list if it goes outside.
Boulder calls do_attack() instead of duplicating.
Adds bouncing boomerang skill. Flappy is ready!