New n-dimensional scatter plot example with GPU-based smooth projections

To-do list:

* [x] minor simplification in `OrthogonalPath`
* [x] drop dependency on sklearn by including the dataset in vispy (#389).
* [x] ~~support for n-dimensional data points (instead of 4 dimensions max here), related to #390~~ (to be done later)

Close #389.

Not on my machine (numpy 1.8.0).

```python
>>> type(load_iris())
numpy.lib.npyio.NpzFile
```