Fast RGB2HSV

Flexible, fast and memory-efficient extension of RGB2HSV

In addition to being inherently more efficient, it introduces optional arguments
- to calculate in single precision
- to calculate only hue or saturation
or value (or combinations of them)

% Checking performance on a Pentium M 1.4:
img = imread('peppers.png');
tic, hsv_old = rgb2hsv(img); toc
% --> Elapsed time is 0.414767 seconds.
tic, hsv_new = rgb2hsv_fast(img); toc
% --> Elapsed time is 0.190769 seconds.