Add Minecart

Added interface for Vehicle and minecart.
Vehicle can be used for boats as well, it allows you to apply a velocity to the entity, which is what vehicles use to move, unlike regular entities.
Cutting from MCP sources:
```
EntityMinecartCommandBlock.java
EntityMinecart.java
EntityMinecartTNT.java
EntityMinecartFurnace.java
EntityMinecartChest.java
EntityMinecartEmpty.java
EntityMinecartContainer.java
EntityMinecartHopper.java
```
Yes, it is an entity per functional block.

The other ones are...interesting.