For API consistency, devs should be able to use a Clone method on ZPT documents

Just as a ZPT Element has a clone method, so should documents.

Example of the element's API (within `IZptElement` interface):
```csharp
/// <summary>
/// Clone this instance into a new Element instance, which may be manipulated without affecting the original.
/// </summary>
IZptElement Clone();
```

This needs to be extended to `IZptDocument`.