Added a test data file of the yahoo format.
	new file:   data/test.yahoo
Modified the code to use the new chivector class that incorporates a header,
which allows fully dynamic feature handling.
	modified:   src/evaluation_measures.hpp
	modified:   src/input_formats.hpp
	modified:   src/lambdarank.hpp
	modified:   src/lambdarank_optimize.hpp
	modified:   src/ltr_algorithm.hpp
	modified:   src/ltr_common.hpp
	modified:   src/ml/linear_regression.cpp
	modified:   src/ranknet.hpp
	modified:   src/ranknet_lambda.hpp