Better label placement for polygons

Using the y-coordinate of the polygon's bounding box, this
simple algorithm intersects the polygon with the horizontal
center line of its bounding box. The x-coordinate of the label
point is the center of the longest segment of this intersection
that is inside the polygon.
Renaming method to getInteriorPoint

as suggested by @tschaub. Also giving credit to JTS.
Merge pull request #878 from ahocevar/polygon-labels

Better label placement for polygons