(fix docs warnings)

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.

Enable travis and other fixes

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).

This reverts commit 2fb9cac0.