Add `Matrix(I, n, m)` method

[andyferris]

In Julia v0.7 you don't need to specify the eltype of the matrix - it is inferred from the UniformScaling{T} input. Compat has only defined the Matrix{T}(::UniformScaling, n, m) method.

# Julia v0.6.2
julia> using Compat

julia> Matrix(I, 4, 4)
ERROR: MethodError: no method matching Array{T,2} where T(::UniformScaling{Int64}, ::Int64, ::Int64)
Closest candidates are:
  Array{T,2} where T(::Compat.Uninitialized, ::Any...) at string:5
  Array{T,2} where T(::Integer, ::Integer) at sysimg.jl:105

julia> Matrix{Float64}(I, 4, 4)
4×4 Array{Float64,2}:
 1.0  0.0  0.0  0.0
 0.0  1.0  0.0  0.0
 0.0  0.0  1.0  0.0
 0.0  0.0  0.0  1.0

[fredrikekre]

#438

[andyferris]

Well we can't do much in Compat about a breaking change to the element type of I (we could define Compat.I I suppose).

The usage I was intending were things like Matrix(4.0*I, 4, 4). I changed these to Matrix{Float64}(4*I, 4, 4) which works on v0.6 (with Compat) and v0.7.