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.

Collision box is 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.

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.