add weights keyword argument; let extents be fractions

Just discovered this *sick* module, and now wishing I hadn't written my own...

This PR adds the ability to weight the samples. This is useful when the samples come from something like nested sampling.

The other thing it does is perhaps more controversial and I could take it out: It lets you pass floats to the ``extents`` keyword. If a float ``f`` is passed, the bounds are automatically set to include the central fraction ``f`` of the samples. (Again, this is useful for nested sampling where the samples might span a huge range but you want to zoom in on the samples that actually have significant weight.)