Separate types for rFFT and FFT plans (and issues with ScaledPlans)

Since PR https://github.com/JuliaMath/AbstractFFTs.jl/pull/58 is now merged, I would suggest talking about how to extend this to `Plans` as well.

One way of doing that would be to implement different (abstract) types for a FFT, rFFT and r2r. This of course would have some implications for the libraries relying on AbstractFFTs. That's why I did not directly proceed with a PR, but would want to discuss this first.

Something reasonable could be to have an `AbstractFFTPlan`, `AbstractrFFTPlan`, `AbstractR2RPlan` that are all subtypes of the current `Plan` type, so that it is not a completely breaking change. (Ideally Plan should be renamed to AbstractPlan, but this would break a lot, I guess)

Aside from this, there is also https://github.com/JuliaMath/FFTW.jl/issues/182 , so that ScaledPlans miss the `region` subfield, which could be mitigated by a function like

```Julia
region(p::Plan) = p.region
region(p::ScaledPlan) = region(p.p)
```