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.
This should be [PRed from vbuterin at pybitcointools](https://github.com/vbuterin/pybitcointools/compare); no need to test here. We trust pybitcointools, but if you want to improve it with tests, that's excellent. However, they don't have a place here.