Make casper.py data structure agnostic

[naterush]

Issue

Currently, casper.py must change depending on the data structures on which consensus is being formed. This makes it harder for binary + blockchain + possible re-write proof stuff to live in the same branch as it will lead to a fair bit of code duplication.

This is one of the considerations for issue #26

Implementation

Change casper.py to be data structure agnostic by having Casper deal mostly with message propagation and generation. Then, the resulting view (and validator set?), as well as the most recent messages propagated, can be passed into some plot tool, which can display them in the regular view graph.

Without this refactor, the plot tool will have to change in any case - but this reduces the amount of code that has to change and/or duplicated for multiple data structures to exist together (and this likely makes it easier to change).

[naterush]

#65 <- this issue supersedes. Closing this.