add ocl::abs and ocl::max for CV_32FC1

This is a first implementation of abs() and max() functionality. In the current state, it is rather restricted to the special case that I needed. However, it does work.
If it is acceptable as such, I'll try to add a test case. It should be rather straightforward to adapt it to other image types and depths. Finally, performance optimization (vectorization and others) could be another future task.