fixes imports in __init__.py and test_echo.py

This PR fixes the internal imports from `echo.py`.

Before:

```sh
$ python -c 'import pyecho ; print("success!")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/private/tmp/echo/pyecho/__init__.py", line 1, in <module>
    from echo import echo
ImportError: No module named 'echo'
```

After:

```sh
$ python -c 'import pyecho ; print("success!")'
success!
```