Fixes units behind stairs blocking movement

Have a unit underneath the skyranger like so
![screen001](https://f.cloud.github.com/assets/1824834/1337147/da34da52-35d1-11e3-8e5b-fdcfb2d2e9c0.png)

The unit on the ramp is unable to reenter the skyranger!
![screen003](https://f.cloud.github.com/assets/1824834/1337155/0492c30e-35d2-11e3-922e-cef17a77c21a.png)

unless you move the selected unit (Tudor Serban here) out of the way.

Similar situation with a Lightning - unit on stairs cannot reenter because of unit behind stairs
![screen004](https://f.cloud.github.com/assets/1824834/1337170/4eb788b6-35d2-11e3-9f4e-661bc1634cec.png)

In a barn, the problem is unit cannot go forward .. 
![screen005](https://f.cloud.github.com/assets/1824834/1337188/916abf8e-35d2-11e3-81b5-f7388b0720eb.png)

.. first has to go East
![screen006](https://f.cloud.github.com/assets/1824834/1337197/ba7b792c-35d2-11e3-8db3-b13e597657c9.png)

E-W Stairs in town is like barn stairs
![screen007](https://f.cloud.github.com/assets/1824834/1337225/308156a0-35d3-11e3-8fc2-3096b3d6fa7f.png)

In this case it's a Snakeman blocking.  If the unit going up the stairs, in this case Salvador, in unaware of the enemy unit then stair movement is unaffected.

N-S stairs in town is like E-W stairs
![screen008](https://f.cloud.github.com/assets/1824834/1337244/8ffae614-35d3-11e3-8e0d-5421ec6139e4.png)

Have not tried any other stairs.

As there is an existing isOnStairs method, I propose
- extending it to handle skyranger ramps 
- to use it when checking if destination is blocked when pathfinding