Macro shorthands for sampling

[Roger-luo]

Some shorthands can be provided for samplers to simplify most declaration sampling task. This will make user's code readable and cleaner.

Currently, we have to use the following work flow to define sample tasks (SamplePlan) and proposal distribution is dispatch ed manually to other processors by @everywhere.

space = RealSpace(min=-4, max=4)
sampler = MHSampler(space)

@everywhere function normal(x::T) where T <: Real
    factor = 1 / sqrt(2 * pi)
    return factor * exp(- x^2/2)
end

plan = SamplePlan(normal, sampler)
data = sample!(plan)

By providing macros @sample could make this easier, and multi-processing's function evaluation should be done inside the macro automatically according to given worker pool. The syntax of this macro is to be decided.

[Roger-luo]

@GiggleLiu comments?