Bug 1284

Improve Stochastic function adding slow Stochastic.

From here:
http://ta.mql4.com/indicators/oscillators/stochastic

The Stochastic Oscillator has four variables:

    %K periods. This is the number of time periods used in the stochastic calculation;

    %K Slowing Periods. This value controls the internal smoothing of %K. A value of 1 is considered a fast stochastic; a value of 3 is considered a slow stochastic;

    %D periods. his is the number of time periods used when calculating a moving average of %K;

    %D method. The method (i.e., Exponential, Simple, Smoothed, or Weighted) that is used to calculate %D.

The formula for %K is:

%K = (CLOSE-LOW(%K))/(HIGH(%K)-LOW(%K))*100

Where:
CLOSE — is today’s closing price;
LOW(%K) — is the lowest low in %K periods;
HIGH(%K) — is the highest high in %K periods.

The %D moving average is calculated according to the formula:

%D = SMA(%K, N) 

Where:
N — is the smoothing period;
SMA — is the Simple Moving Average.