Added the LiquidHopper.

As discussed in issue #1291 I implemented the Liquid Hopper.
![2013-11-03_15 56 12](https://f.cloud.github.com/assets/440384/1461478/597fa91e-44b3-11e3-8181-8a6c4bc4527b.png)
(left: Regular hopper, right: liquid hopper)

This block behaves just like the normal hopper, with that difference that only buckets and fluid containers (IFluidContainerItem) can pass trough it. When those items go trough the LiquidHopper, they will be emptied and the liquid will outputted to any connected pipes and/or tanks, while the empty item/bucket will go to the inventory beneath it. The speed of the empty process depends on the viscosity of the fluid. While an item is being emptied, the hopper won't try to empty any other item until the first one is completely empty.

The texture is exactly the same as the normal hopper, with that difference that it's slightly darker. This could be changed, but since I'm no expert in changing models it'd be better if someone else took care of this.

Any remarks are welcome!