code: function getDelta(s, k, t, v, r, callPut)
  {
    if(callPut === "call")
    {
      return _callDelta(s, k, t, v, r);
    }
    else // put
    {
      return _putDelta(s, k, t, v, r);
    }
  }
docs: Calculates the delta of an option.
  
  @param {Number} s Current price of the underlying
  @param {Number} k Strike price
  @param {Number} t Time to experiation in years
  @param {Number} v Volatility as a decimal
  @param {Number} r Anual risk-free interest rate as a decimal
  @param {String} callPut The type of option - "call" or "put"
  @returns {Number} The delta of the option