Added streamplot method to GeoAxes.

This PR adds the final piece in the vectors puzzle: streamplot.

Streamplot is non-trivial because the calculation of the streamlines requires vector components on an evenly spaced grid. This implementation uses the new vector transforms utility to regrid the vector field in projection space, and plots in native coordinates.

There are a couple of locations where workarounds for deficiencies in matplotlib's streamplot are desirable, but I hope they aren't too unsightly!