[QRS detection] OpenMP introduced

* Change:
  - Median and box filter are the most consuming parts
    of the program, so they are parallelized with OMP.

* Note:
  - On Intel(R) Core(TM) i5-3210M CPU with two physical
    cores speedup are showen below:
      - BoxFilter    2.28x speedup
      - MedianFilter 2.85x speedup
      - Total time   2.50x speedup

Signed-off-by: Dusan Kostic <kostic.dusan1990@gmail.com>