Extend blob tracker to work with color

[fspindle]

There exists the vpDot2 class that implements a blob tracker. This implementation is only working with grey level images vpImage<unsigned char>. It could be useful to extend the algorithm to work on RGB color images vpImage<vpRGBa> and why not on HSV images when this color format will be introduced (see #496).

It should be possible to introduce this new feature making vpDot2 as a template.

[yshkp]

@fspindle Will making the RGBa container and removing the conversion of the frame, with their gray level work ? and in vpDot2.h , How setGrayLevelMax and setGrayLevelMin is defined ?