Add copyright info to Feature.
Fix index error of vector in omenquire.cc.
Update the type of variables in Ranker.
Update the type of DEFAULT_NORMALIZER.
Fix bug in read_problem() of SVMRanker.
Update SVMRanker.
Update RankList.
Fix bugs in get_feature_values_text().
Add new functions for outputting feature info.

Add get_feature_values_did_text and get_label_feature_values_did_text.
Update get_label_feature_values_text to accept argument qid.
Remove Normalizer * from Letor::Internal.

Add new function set_normalizer in Letor::Internal to set normalizer in
FeatureManager.
Update read_from_bin and read_from_txt.

Use write_to_txt as default for preparing training data.
Fix bug in init() and use read_from_txt as default.

Change the output format of prompt.
Fix bugs in set_ranker and set_normalizer.
Update the constructor and add destructor.
Update the constructor of Letor::Internal.
Update the type of flags for ranker and normalizer

Use '\n' instead of std::endl.
Change the output format of prompt.