Add missing methods for UniformScaling

[jishnub]

The methods float, cis, sincos and sincosd are defined for matrices, so it makes sense for these to be defined for a UniformScaling as well.
E.g.:

julia> float(2I)
UniformScaling{Float64}
2.0*I

julia> sincos(2I)
(UniformScaling{Float64}(0.9092974268256817), UniformScaling{Float64}(-0.4161468365471424))