move_to, look_at, marker and selection

Changelog:
 1. move_to:
  - Moves to the location desired.
  - Uses angle to locate the position to translate to.
  - First "looks at" the target and then moves to that direction.
  - Field 'target' is the established position to be moved to.
 2. look_at:
  - Looks at the direction given.
  - Both returns and sets the field 'angle'.
 3. Updating move_to:
  - body:update handles the checking whether the body has reached his
    destination.
  - Distance ranging from 0 to 3 pixels.
 4. Selection:
  - Pressing left mouse button and dragging will allow the user to select
    bodies.
  - Selected bodies will be outlined blue.
 5. Marker:
  - When the player presses the right button, a marker will be positioned
    telling where the bodies will go to.
  - The marker will disappear once at least one body has reached its
    destination.