Move example data files to `pyregion.datasets`?

[cdeil]

Should we move the example data files that are used in the docs (e.g. docs/figures/pspc_skyview.fits or docs/figures/test.reg) to pyregion.datasets to make the docs examples work for the user out of the box?

@astrofrog If you agree, this is something I could do tomorrow, it's not much work.