PolygonalBasinGravity for 2D gravity inversion

Added a new class for 2D gravity inversion. Parameterizes the basin
relief by a polygon and estimates the z-coordinates of the vertices.
Cookbook recipe for polygonal basin inversion
Changelog entry for PolygonalBasinGravity
Permit setting model x limits

Need to edit the mesher.Polygon class to make .x and .y attributes into
properties.
Merge pull request #149 from fatiando/basin-polygon

2D gravity inversion for the relief of a basin using a polygonal parametrization