Add Rotation for Item Frames

This creates a `Rotation` class, containing the rotation values for 1.8

It also adds `getItemRotation` and `setItemRotation` to `ItemFrame`, to allow getting and setting of the item displayed in the item frame.

**A note about the rotation names**

It might seem odd that I'm using enum values for degrees, instead of somthing like `setItemRotation(180)`.

The reason for this is that items frames use a metadata value (an `int` ranging from `0` to `7`) to store rotation.

Allowing any degree value to be set creates the illusion that this is actually support by item frames. The names I've chosen for the enum value reflect what I think makes the most sense for the defined rotation values. I'm open to any suggestions as to improving the naming.