Hello,

I would like to define two distributions:

```
p = Uniform(0,1)
p2 = 1-p
```

give me typrerror.

ERROR: MethodError: no method matching -(::Int64, ::Uniform{Float64})

I also cannot sum two distributions:

```
c = Normal(20,5) + Normal(10, 6)
```

I found this Issue: https://github.com/JuliaStats/Distributions.jl/issues/142

Is it possible to build the new distribution in different ways?

Has Turing a better support for operations among distributions?