Starting improve bidirectional integrator.
Separate class declaration code to bdpt.h header file for cleanup code.
Try to add news parameters:
    - maxPathLength
    - minPathLength
Comment unused lines on color.h
Add defines for MinGW32 compilers in y_alloc.h
Small mismatch typo on BDPT_H guard
Add 'do_lightImage' variable for test.
Indent some license text.
Merge remote-tracking branch 'remotes/origin/bidirectional_cleanup'

Conflicts:
	include/yafray_constants.h
	src/integrators/bidirpath.cc