Preliminary result for OpenMP wrapper testing

Hi Ulli,

Please consider my pull request to see the code I have mixed up from Enblend project. I would not expect to merge the code to your branch but It would be nice to hear your comments soon.

In this pull request, I tested function "CombineTwoImages" with OpenMP wrappers. I tested and find that it used three threads to run in parallel.

There are two issues:
-RESTRICT from openmp_vigra.h (Enblend) seems to be not a C++ keyword or my compiler g++-4.8 does not accept this keyword. I temporarily disable this keyword.
-Unfortunately, the function "CombineThreeImages" did not run in parallel as I expected. The function  vigra::omp::combineThreeImages(srcImageRange(img), srcImage(img), srcImage(img1), srcImage(img3), Arg1() + Arg2() + Arg3()); failed while compiling. I still need to learn from your code.

Please consider and give me any comments/suggestion! Thanks,
M.f.G,
Minh
The destination image must be passed with `destImage(img3)` instead of `srcImage(img3)`. See @cspiel's comment.