Remove explicit methods for zero and one

Revert PR #19 (but leave tests)

src/DoubleDouble.jl

@@ -5,8 +5,7 @@ import Base:
     convert,
     *, +, -, /, sqrt, <,
     rem, abs, rand, promote_rule,
-    show, big,
-    zero, one
+    show, big
 
 abstract AbstractDouble{T} <: Real
 
@@ -52,11 +51,6 @@ function splitprec(x::AbstractFloat)
 end
 
 
-zero{T<:AbstractFloat}(::Type{Double{T}}) = Double(zero(T))
-one{T<:AbstractFloat}(::Type{Double{T}}) = Double(one(T))
-zero(a::Double) = Double(zero(a.hi))
-one(a::Double) = Double(one(a.hi))
-
 # ones(T::Double, dims...) = fill!(Array(T, dims...), (one)(T))
 # zeros(T::Double, dims...) = fill!(Array(T, dims...), (zero)(T))