Added support for seed in generating keypair

[excerebrose]

Description

Adds support for seed in generating key pair as in the javascript driver, links to commit in the crypto conditions repository

Related PRs

Linked to change in cryptoconditions repo

Repo/Branch	PR
Cryptoconditions/Master	link

Todos

• [ x ] Tested and working on development environment
• [ x ] Added/Updated all related documentation. Add [link]() if different from this PR

[ttmc]

Hi @excerebrose --- See the comment I made on your related pull request in the cryptoconditions repository: https://github.com/bigchaindb/cryptoconditions/pull/116#issuecomment-431458598

Some tests are failing. There was already a docs test that was failing and we hadn't gotten around to fixing; don't worry about that one. Besides that, we'll also have to get your cryptodonditions pull request merged first, then release a new version of the cryptoconditions package, then modify this pull request to use the new cryptoconditions package. That should fix the other failing tests.

[codecov-io]

Codecov Report

Merging #487 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #487   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files           5        5           
  Lines         235      235           
=======================================
  Hits          232      232           
  Misses          3        3

[ttmc]

@excerebrose I updated cryptoconditions to version 0.8.0 and fixed a small Flake8 error. There's still one test failing but it's a previously-known issue with the docs, which can be handled by another, future pull request (so don't worry about that). Please address my first comment about the docstring. (My second comment was wrong and I marked it as resolved.) Once that is done, I should be able to merge this pull request.

[excerebrose]

@ttmc Fixed the docstring.you should be good to go