A new python sample on grabcut

* A new python sample on grabcut (grabcut.py) is added. 

* Usage : python grabcut.py [image]
   * Then follow instructions on the terminal.
   * Some results can be seen here : https://github.com/abidrahmank/MyRoughWork/tree/master/blog_works/grabcut

* A small typo in grabcut docmentation is corrected.
* Solved Bug #2960 ( No mentioning of CV_BGR2GRAY in cvtColor docs )
* **Issue** : In python, we have to specify the size of 'bgdModel' and 'fgdModel', which is not  specified anywhere. But it is not needed for C++. Is it better to mention it in docs or solve the issue in the code?