Initial cleanup.

* Drop unused code.
* Fix import style.
* Abstract the Elliptic Curve used.
* Suggest to replace any occurences of 'secp256k1' in rest of codebase
  with crypto_util.BTC_CURVE
Use BTC_CURVE in test/test_ec.py.
Use BTC_CURVE in node/transport.py
Use BTC_CURVE in node/connection.py.
Refactor pubkey_to_pyelliptic.

* Tag key variables with number base.
* Avoid destructive updates, so that code flow is super-clear.
* DRY key processing.
As principle, never output keys to stdout, unless necessary.
DRY pubkey processing (part 1)
DRY pubkey processing (part 2)
Refactor makePubCryptor, hexToPubkey, makePrivCryptor.

* Tag key variables with number base.
* Avoid destructive updates, so that code flow is super-clear.
* DRY key processing.
More DRY key processing (part 1).
More DRY key processing (part 2).
Helper function should be private to module.