Drop node/arithmetic.py

### Changelog:
* Unit test a small part of `pybitcointools` API.
* Use `pybitcointools.main` instead of `node/arithmetic.py`.
* Drop `node/arithmetic.py` and `test/test_arithmetic.py`. See #590.
After looking at it, it seems that some refactoring of the pybitcointools testsuite to use `unittest` would be appropriate. I recommend that you remove the test code from here for now so that we can merge, and we worked together with Vitalik to refactor their tests into unit tests and include your own test methods.