New feature to tracking sample

Now initial bounding frame can be given in command-line, as an alternative
to manual selection via mouse. Bounding frame is given as optional last
argument in the format "x1,y1,x2,y2" where x's and y's are integers,
symbolizing bounding box with opposite vertices (x1,y1) and (x2,y2).
Merge pull request #18 from nailbiter/feature

New feature to tracking sample
Skeleton for Median Flow

So far it only displays the constant bounding box. The code compiles.
Inner commit
Inner commit
First draft of Median Flow tracker

The median check is performed only with forward-backward error, without
NCC at this moment.

On synthetic sequence (randomly-colored square) annoying constand drift
is observable. Tests on real sequences were not performed yet.