Abstract the Elliptic Curve used.

* Introduce crypto_util.BTC_CURVE as the curve's name. Replace any
  magic strings in the codebase with crypto_util.BTC_CURVE.
* Invoke pyelliptic.OpenSSL to get curve ID
* Abstract the key length.
Replace hexToPubkey with pubkey_to_pyelliptic.
Test public and private key processing.
Abstract public and private key processing.