Testing Commands in README.md

Tests describe running the command:

``` bash
python ecdsa/test_pyecdsa.py
```

However, I received an error:

``` bash
Traceback (most recent call last):
  File "test_pyecdsa.py", line 11, in <module>
    from .six import b, print_, binary_type
ValueError: Attempted relative import in non-package
```

It worked when running the command:

``` bash
python -m ecdsa.test_pyecdsa
```
