Change Rect to Rect2d in Tracker::update() and ::init()

I've changed Rect to Rect2d in Tracker::update(), Tracker::init() and
all related methods (including documentation). This allows to initialize
trackers with double-valued rectangles, thus adding versality. Besides,
trackers also can output double-valued rectangles, which may be
beneficial in some scenarios.

However, it remains to change UML diagrams in documentation to tracker
module, as they still display methods above with old signatures.
Merge pull request #19 from nailbiter/Rect2d

Change Rect to Rect2d in Tracker::update() and ::init()