[WIP] Edward 2.0

[dustinvtran]

This PR is a redesign of Edward for version 2.0. It adds/changes the following:

1. Core function for manipulating continuations: call_with_manipulate.
2. Inference algorithms are now pure functions (up to your usual TF side effects).

Details and features of design forthcoming.

@csuter, @davmre, @derifatives, @jvdillon, @matthewdhoffman, @mattjj

Breaking changes to API

• Model and variational programs are callables.
• Inference classes only have a functional equivalent and are Tensor-in Tensor-out.
• All edward.criticisms is removed.
• All edward.util is removed.
• All custom Edward random variables are removed: DirichletProcess, Empirical, ParamMixture, PointMass.
• rv.value() is renamed to rv.value.

todos

• Break up into smaller, self-contained PRs.
• Refactor all inferences (excluding Monte Carlo) to support traces.
• Refactor Monte Carlo (waiting on @jvdillon for go/hmc-new-interface)
• Refactor inferences/conjugacy/ (waiting on @matthewdhoffman, @mattjj)
• Add programmable docstrings.
• Update criticisms module.
• Update util module.
• Update tests/. (only tests/inferences/ left)
• Update examples/, docs/, notebooks/.