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.

Sorry, one more comment -- it's cleaner to do `from ._datasets import iris`. The current incarnation will mean that there is `vispy.datasets.np`, `vispy.datasets.get_data_file` (which isn't so bad I guess), etc. The suggested change will make tab-completion cleaner.