Updates traincascade

After performing this pull request : https://github.com/Itseez/opencv/pull/1844
I considered adding some debug information to the training stage of the traincascade.

1) Added the output of sample grabbing at negative and positive samples. (line 324)
2) Added the output of training time after each stage expressed in days, hours, minutes, seconds. (line 254)

Guess this should be merged once 1844 gets merged, since they contain both the fix of 1844 to be sure no merging conflicts will occur later.