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.
You are right, but:
* The functions tested aren't part of `pybitcointools` testsuite.
* The `pybitcointools` [testsuite](https://github.com/vbuterin/pybitcointools/blob/master/test.py) is so complicated I have no idea whatsoever how to PR these tests. :(