Better handling and updated tutorial

Hi all,

An important update:

- much improved ship path following - your ships will follow their target paths much more closely, and won't get completely stuck if heading into the wind and with tack on their own somewhat, though you're still better off plotting  a course yourself that has good margin to the wind.

- an improved tutorial

- fixed zoom handling in the browser - keyboard (+-) and mouse scroll wheel have reasonable zoom steps

Especially the ship path following change significantly improves game play.

Fair winds and following seas, Captains!