code: function getTheta(s, k, t, v, r, callPut, scale)
  {
    scale = scale || 365;
    if(callPut === "call")
    {
      return _callTheta(s, k, t, v, r) / scale;
    }
    else // put
    {
      return _putTheta(s, k, t, v, r) / scale;
    }
  }
docs: Calculates the theta 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"
  @param {String} [scale=365] The number of days to scale theta by - usually 365 or 252
  @returns {Number} The theta of the option