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.
Sure, I can start reworking the test.py in pybitcointools into unittests.

On Wed, Sep 17, 2014 at 10:05 AM, Dionysis Zindros <notifications@github.com
> wrote:

> In test/test_pybitcointools.py:
>
> > @@ -0,0 +1,59 @@
> > +import unittest
>
> 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.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/OpenBazaar/OpenBazaar/pull/595/files#r17665285>.
>