UniformScaling: allow writing generic polynomicals with I.

```julia
julia> p(x) = x^2 - 2x + I
p (generic function with 1 method)

julia> p(4)
9

julia> p(rand(3,3))
3x3 Array{Float64,2}:
  0.386215   -0.22931    0.0859286
 -0.18856     0.664356  -0.693054
  0.0600554  -0.361883   0.619314
```