Code refactoring: Extract class DistanceMetric from ColorHistogram.
Implement several new distance metrics: Tanimoto, NED, Quadratic, Mahalanobis, PCC.
Fix severe bugs in Chi-Square, Canberra, Wave-Hedges, Mean distance.
Improve performance by ommitting irrelevant calculations in: Euclidean, Manhattan,
Bray-Curtis, Soergel, Histogram Intersection, Chord, Kolmogorov-Smirnoff.