Stop lying about eltype

[kleinschmidt]

The eltype of, for instance, NormalInverseGamma should be Tuple{Float64, Float64}, but instead is given as Float64. This means that rand chokes because it allocates an array of floats instead of an array of tuples:

julia> d = ConjugatePriors.NormalInverseGamma(0., 1., 2, 2)
ConjugatePriors.NormalInverseGamma{Float64}(mu=0.0, v0=1.0, shape=2.0, scale=2.0)

julia> rand(d)
(-0.6216462947065958, 0.7004872075936253)

julia> rand(d, 2)
ERROR: MethodError: Cannot `convert` an object of type Tuple{Float64,Float64} to an object of type Float64
This may have arisen from a call to the constructor Float64(...),
since type constructors fall back to convert methods.
Stacktrace:
 [1] _rand!(::ConjugatePriors.NormalInverseGamma{Float64}, ::Array{Float64,1}) at /home/dave/.julia/v0.6/Distributions/src/genericrand.jl:37
 [2] rand(::ConjugatePriors.NormalInverseGamma{Float64}, ::Int64) at /home/dave/.julia/v0.6/Distributions/src/univariates.jl:182

julia> eltype(d)
Float64