Model defined for Block

[barnjamin]

Problem

A number of folks have started working with the blocks from an API. We don't have any good models defined for them so anyone that wants to work with them needs to define them on their own.

Solution

Build out a class that can take a msgpack/json response from the API and provide utility methods for getting instantiated Transaction objects to work with them.

[barnjamin]

Wrote something dirty for parsing out the transactions from the block, does this kind of thing belong in the SDK? https://github.com/barnjamin/sdk-extras/blob/master/py/block_fetcher.py

[algoanne]

related, to discuss: #479