Fix d3.geo.{guyou,peirceQuincuncial}.

Recent optimisations in Chrome 33 have brought an issue to light with
the use of sgn(x) in d3.geo.guyou, in situations where x = 0.  This is
because sgn(0) = 0, and in d3.geo.guyou we required the “sign” of 0 to
be defined as +1 instead.

Fixes #10.