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

Not ready for merge, but you can already try it!

Things to be done:

* [ ] minor simplification in `OrthogonalPath`
* [ ] support for n-dimensional data points (instead of 4 dimensions max here), related to #390
* [ ] drop dependency on sklearn by including the dataset in vispy (#389)

So let's do `from vispy.datasets import load_iris` -- you'll need to add the appropriate files to https://github.com/vispy/demo-data. +1 for making it part of this PR, since it's a good use-case for it.