removed wildcards from fg/bg path in ICFDetector::train

hardcoded .png for positives and .jpg for negatives are a useless restriction, users should supply their own wildcard arguments (if any)

apart from that, imho it would be desirable, that the train method takes another argument, a string with the feature_type to be use ("icf" or "acf", currently hardcoded internally to "icf") , and that it returns the highest threshold level obtained from waldboost (a good threshold value seems to be essential for the prediction later), 
but that would break the current interface.