Basic support for weighted heatmaps.

Can now have a 3-column data list to represent (longitude, latitude,
weight).
Added error messages for incorrect data arrays.

Passing in the wrong data array now gives a sensible error message.
Also checks that all the weights are positive.
Added documentation on weights to the heatmap docstring.
Merge pull request #33 from pbugnion/weighted_heatmap

Allow weighted points on heatmaps.