Deprecate node/arithmetic.py

### Motivation:
The [node/arithmetic.py](https://github.com/OpenBazaar/OpenBazaar/blob/master/node/arithmetic.py) module contains a lot of number-theoretic code, but only 2 of its functions are used in the rest of the repo. Both of these can be replaced by functions of the [pybitcointools.main](https://github.com/vbuterin/pybitcointools/blob/master/bitcoin/main.py) module:
* `arithmetic.changebase` with `main.changebase`
* `arithmetic.privtopub` with`main.privkey_to_pubkey`

We should not implement our own crypto, if possible. This PR adds some tests to document the redundancy.

### Changelog:
* Document the redudancy of `node/arithmetic.py` via unit tests against `pybitcointools.main`.
* From this point onwards, `node/arithmetic.py` is considered *deprecated*. Unless there is some objection, there will be a follow-up PR replacing the calls as noted and __removing__ `node/arithmetic.py`.
Nice, I hope you don't have any bitcoins stored there ;)