fix constant addition

due to
julia> s = 1.0 + SinOsc()
ERROR: `convert` has no method matching convert(::Type{AudioNode{T<:AudioRenderer}}, ::Float64)
 in OffsetRenderer at /Users/goretkin/.julia/v0.3/AudioIO/src/nodes.jl:178
 in + at /Users/goretkin/.julia/v0.3/AudioIO/src/operators.jl:16