Add presimplify and resimplify.

To facilitate interactive simplification, you can now call presimplify first,
which computes the effective area of each point. Then, you can call resimplify
to derive a filtered topology from the presimplified topology. This returns a
new object, and thus can be called repeatedly for fast interactive filtering.
Use point[2] instead of point.area.
Enable dynamic simplification in the client.

A new topojson.presimplify method, available in the client library, computes the
importance of each point using a user-specified function, defaulting to the
Cartesian triangular area (for Visvalingam simplification). This defines the z-
dimension of each coordinate in the topology, allowing the topology to be
filtered dynamically during rendering.
Delete cruft.
Fix for removed points in delta encoding.
Oops, fix output.
Ensure cartesian areas are positive in pixels.

When we say "Cartesian" we really mean screen pixels, in which case the
vertical y-axis origin is at the top, not the bottom as in conventional
mathematical notation.  This affects the sign of the area calculation.

Forcing clockwise winding order in screen pixels requires us to change
the area sign to ensure that positive areas mean clockwise winding order
for both spherical and Cartesian coordinate systems.
Merge branch 'cartesian-area' of https://github.com/jasondavies/topojson into resimplify
Fix tests for Cartesian order change.
Merge branch 'resimplify'