Boats: fix and improve y motion and turning, smaller collision box for diagonal motion

Testing boats mod in the extreme waterfalls/rapids of my 'canyon' mod has shown that y motion is not good, this is my fault and due to my previous work on boats mod. The problem with y motion is not obvious with flat water or gently rising rivers.

Collision box is a little narrower, to fit the boat when diagonal, to avoid unnecessary collisions when diagonal.
![screenshot_3724866702](https://cloud.githubusercontent.com/assets/3686677/4517450/32a94ff8-4c36-11e4-88bb-1bf31c27765f.png)
Of course this makes the box narrower than the boat when axis-aligned, but i feel this is less irritating than a collision caused by a non-existant part of the boat.

I made decelleration 0.1, the same as acceleration, also for identical acceleration when driving in reverse.

Cleaned up and simplified turning code, added 'elseif's and removed the reversed turning when decellerating/reversing. Now the boat can be driven in reverse and the turn controls act properly. Currently when you hit the brakes, the turn controls are reversed.

'self.object:setpos(self.object:getpos())' has been added in 4 places, working on my 'mesecar' mod i discovered this makes turning much smoother, i don't know why.

I discovered that the increased y acceleration when underwater was not working because it was being overriden by the 'set acceleration' at the end of the function, this is now fixed.

Maximum y veocity is changed to 4.5 to match the maximum horizontal velocity, enabling fast boat travel up very high waterfalls (which could be used as a vertical route).

Y accelerations while in water and underwater have been fine tuned for a bouncy, floaty, realistic feel to the boat when travelling through extreme changes in water height, please test in my 'canyon' mod to experience this.

Finally, the y velocity threshold that triggers the boat to be repositioned at the surface at zero y velocity, is changed back to the old value of 1, to make the boat bob more when travelling water slopes, it still settles down soon after to a zero y velocity on flat water.