Add type promotion

[dlfivefifty]

The following should return a Vector{Fill{Float64,2,Tuple{Base.OneTo{Int64},Base.OneTo{Int64}}}}:

julia> [Zeros{Int}(2,2), Ones(2,2)]
2-element Array{FillArrays.AbstractFill{Float64,2,Tuple{Base.OneTo{Int64},Base.OneTo{Int64}}},1}:
 [0.0 0.0; 0.0 0.0]
 [1.0 1.0; 1.0 1.0]