Use functors for ground distance in Earth Movers Distance (EMD) computation.

Original sources for emd() use a function pointer to allow different ground
distance implementations. Switch to using functors instead.
Move code for Earth Movers Distance (EMD) computation to vigra/emd.hxx.

Source code available with the Rubner, et al. paper [1] was imported into
/test/emd/emd.*. Now that it works better with the library conventions, it
can be moved to its final place.
Implement earthMoverDistance() in vigra/emd.hxx. Use this function in tests.

Move the contents of the  wrapper emd() function in test/emd/emd.hxx to
fill the contents of earthMoverDistance stubs.
Implement earthMoverDistance functions withour norm arguments.

The norm is selected based on the feature type by template specialization.