Migrate tests to pytest

[naterush]

Issue

• Currently, we use unittest as our unit testing framework, which is limiting in terms of using command line arguments w/ tests.
• lengthy self.assertEquals statements are lengthy :)

Proposed implementation

Move to pytest as our unit testing framework. Should be a simple enough refactor - but will allow for better command line arguments and cleaner tests.

Subtasks...

• add command line arg to enable plotting test graphs

[djrtwo]

pytest's parametrize feature will be particularly useful to us. It allows you to really easily use the same code for multiple versions of the test. For example, if we wanted to run test_is_in_blockchain against a bunch of different test_strings.

See here for a sample usage in web3.py

[djrtwo]

I'm picking this up.

[naterush]

Done here: #30