Added Tile:getPosition() method

I believe the Lime is missing this method for tiles.
In my case I want to set one tile's position to another tile's position. They are on different layers that have matching positions so this is ok.
I can't use
```lua
one:setPosition(another:getWorldPosition())
```
because getWorldPosition is affected by map dragging.
With this change I can do what I want:
```lua
one:setPosition(another:getPosition())
```