Generic reader

See old issue 14 ;)
Done on 2D/3D, nD (not tested with raw reader).
As proposed by David we can now do:
MyImage image = GenericReader::import("glop.pgm");
MyImage image = GenericReader::import("glop.vol");

In the same way I define a generic writer:
image >> "glop.vol"

(just tested with ImageContainerBySTLVector and there are some issue with ImageContainerBySTLMap but not related to this work I think, I will put an issue later).