add class IotaBlock to python binding

[mp-v2]

N.B. this PR has been replicated as per feedback into the new ioa-sdk repo, see new PR here:
iotaledger/iota-sdk#229

Description of change

This PR adds to the python binding to make encoding data to byte format and then to hex format easier.
It adds the class IotaBlock in the same structure as the IotaClient class, and uses similar terminology to existing python binding example scripts.

Type of change

Choose a type of change, and delete any options that are not relevant.

• Enhancement (a non-breaking change which adds functionality)

How the change has been tested

Used the class, ran some different tests manually.
Make sure to provide instructions for the maintainer as well as any relevant configurations.
python3.5 or higher needed for the hex conversion builtin functions of python

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes