Allow non-visual environments

[Kaixhin]

As an enhancement to #26, it would be great for the network to take in any kind of state as specified by the rlenvs API, as long as the state is a single tensor. This is slightly restrictive, but with the -modelBody option a lot can still be done.

Major changes would be to a) make visual preprocessing more explicit as options and b) have environments output a 3D array as part of a getDisplay method so that visuals can still be provided. An option also needs to added to discretise/not discretise the experience replay memory.

A "roadmap" can be seen in the readme of the nonvis branch.