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.
@dionyziz
I suggest we stay the merge until the `pybitcointools` tests are in place. 

@vbuterin Expect PRs :)