Remove vegan3d functions

**vegan3d** package works now and contains all **rgl** and **scatterplot3d** based functions of **vegan**. If we are going to split that as a separate package, we should remove those functions from **vegan**. This PR does so -- as a response to issue #29. However, this also moves `ordiplot3d` in addition to **rgl** functions. However, **vegan3d** separates the doc files so that moving `ordiplot3d` back to **vegan** is now easy if judged sensible.

This also changes dependencies (`DESCRIPTION`, `NAMESPACE`). This means potential conflicts with any other changes touching those files.

This PR may require a couple of days of thinking before merging, and broadcasting to the **vegan** team.