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 a new area, StraightLine

Adds a new area, StraightLine returns a straight line center on general
Merge branch 'mechanics' of https://github.com/Alberdi/rogueforce into sito
Adds a new General on saviours faction

Adds a new general, Archer, with 2 skills.