Correcting bgsegm module descriptions.  (#493)

* Correcting bgsegm module descriptions. The algorithm implementation doesn't have multi target tracking as mentioned in original paper. it only does foreground/background segmentation.

* Removing opencv_ from heading

Removing opencv_ from description

- made some of dependencies explicit
- removed dependencies to highgui and some other modules where possible
- modified some samples to build without modules

* added BackgroundSubtractorCNT

* doc fixes

* fixed typo