Add a snap_to_grid method on everything.

Based on the PostGIS ST_SnapToGrid function.
Add geometry transformations, like rotate, scale, etc.

Based on the PostGIS transformation functions.
Add a Geos::LineString#line_interpolate_point method.

Based on the PostGIS ST_Line_Interpolate_Point function.
Update the list of Rubies known to work.
Clean up the README to include a note about new methods rather than a list.
Add a note about the new `Geos::Error` exception class.