Render the distributions in Distributions.jl compatible with automatic differentiation in ForwardDiff.jl

• This requires that each distribution d derives from Abstract<d>.
• We create a parameterized, differentiable distribution corresponding to each d. Thus, for the case of Normal <: AbstractNormal, we have:

immutable DNormal{T <: Real} <: AbstractNormal
    μ::T
    σ::T
end

• We then extend Normal with an outer constructor that, when any argument x::ForwardDiff.ForwardDiffNumber, delegates to a constructor for DNormal.
• This allows Normal to continue as a leaf type while allowing differentiability to be facilitated transparently through parameterized distributions.