See https://github.com/Itseez/opencv/issues/5721

COMMENTS:
* The second __syncthreads() is necessary, I am sure of that.
* The code works without the first __syncthreads() too, but I have however added it for symmetry. Anyway it doesn't affect time performances, I have checked it with some profiling with nvvp

changed hog to work with variable parameters and changed the hog sample to test it with more options

added comments and tests

use abstract interface with hidden implementation

move GPU constants initialization from constructor to compute method