Fix a crash in the IndependentMultimodal algorithm during setup. The … (#108)

* Fix a crash in the IndependentMultimodal algorithm during setup. The nframes member
    variable is not initialized, which means that occasionally the initialize() function
    is not called when the first frame is processed.

    * package_bgs/IMBS/IMBS.cpp
      (BackgroundSubtractorIMBS): Initialize nframes to 0.

* Fix a crash when initializingn not using and the destroying the IndependentMultimodal algorithm.

* package_bgs/IMBS/IMBS.cpp
  (BackgroundSubtractorIMBS): initialize bgBins, bgModel and persistenceMap pointers to NULL.