Offline/mock unit tests

[AndreMiras]

Make unit tests pass even being offline using unittest mock.patch.
This would also make Etherscan calls more reliable in both local testing and Travis testing.

[AndreMiras]

List of test failing without connection:

1. src/tests/test_pywalib.py::PywalibTestCase::test_get_balance
2. src/tests/test_pywalib.py::PywalibTestCase::test_get_balance_web3
3. src/tests/test_pywalib.py::PywalibTestCase::test_get_nonce
4. src/tests/test_pywalib.py::PywalibTestCase::test_get_nonce_no_transaction
5. src/tests/test_pywalib.py::PywalibTestCase::test_get_out_transaction_history
6. src/tests/test_pywalib.py::PywalibTestCase::test_transact
7. src/tests/test_pywalib.py::PywalibTestCase::test_transact_no_funds
8. src/tests/test_pywalib.py::PywalibTestCase::test_transact_no_sender