Minor code cleanup.
Avoid sharing points.

This can happen when reprocessing TopoJSON input (type=Topology), where
a two-element array instance (point) was being used more than once in
some situations to obtain a valid line or polygon geometry.  This caused
shared points to be quantized multiple times, resulting in large and
incorrect coordinates.
Remove zero-area interior rings.

Also, don't remove polygons with a zero-area exterior ring, if they have
non-zero interior rings.
Minor optimisation: remove unused code.
Merge branch 'zero-area' of git://github.com/jasondavies/topojson into 1.1.5
Merge branch 'shared-points' of git://github.com/jasondavies/topojson into 1.1.5
Update example files.
Merge branch '1.1.5'