added samples to use retina in its basic form and for HDR tone mapping
updated code formating of reina samples
More abstract tracker declaration

This contribution changes the way how trackers should be implemented by
programmer. It should be noted first, that from the user prospective,
Tracker::create() method still can be called with the same effect.
However, there's no more thing as a "constructor" for Tracker, they
should be created via static functions, e.g.
    static Ptr<TrackerMIL> createTracker(const TrackerMIL::Params &parameters=TrackerMIL::Params());

From the programmer's perspective now for every tracker what is declared
in tracker.hpp header is more a "interface", while the real
implementation class can (and has to) be implemented in .cpp file.

Changes in documentation are also included.
Merge pull request #30 from nailbiter/abstract

More abstract tracker declaration
Merge pull request #24 from albenoit/master

added samples to use retina in its basic form and for HDR tone mapping
fix compiling reg module after separating imgcodecs from highgui
Merge pull request #33 from vpisarev/reg_imgcodecs_fix

fix compiling reg module after separating imgcodecs from highgui