code: function sign(private_key, data) {
    var key = new bitcoinjs.ECKey(new BigInteger(private_key, 16), false);
    var hash = bitcoinjs.crypto.hash256(new Buffer(data, 'hex'));
    return key.sign(hash).toDER().toString('hex');
  }
docs: Sign the given data
  
  ** For now assumes ec_secp256k1 **
  
  @param {String} private_key Hex-encoded private key
  @param {String} data Hex-encoded data