Fancy declarative syntax for a subset of problems

In talking with @odow at Juliacon, I started thinking that POMDPs.jl is still quite difficult to use because much knowledge must be assimilated before even getting started, and much functionality (notably discretization) is still missing. This is especially true for people new to julia or engineers who are not that experienced at programming. Many people who start using this software don't even really know what an interface is and, even if they do, Julian interfaces look much different than they do in other languages, so, before they can use the package, they must learn about Julian interfaces.

One partial solution to this would be to create a JuMP-esque declarative syntax for specifying a subset of problems (e.g. maybe we could even handle all MDPs?). It might look something like this https://github.com/odow/SDDP.jl/blob/master/examples/FAST/hydro_thermal.jl . For example, there would be a `@state` macro to declare the state space, and `@action` macro for the action space, etc.

The current POMDPs.jl interface could function as a common backend for solvers and problems as MathProgBase currently does.