Add FeatureSelector to letor module.

Partial implemented.
Add std function.

Change the type of argument x_train of kde_pdf to const and reference.
Add new function which calculates kl divergence.
Add compute, fetch, select and new d_js functions.

The compute function calculates weight for each feature. The fetch function
extracts top k features. The select function return the top k features. The
new d_js function calculates JS divergence.
Add read_from_file function.

The read_from_file function restores FeatureVectors from file.
Add extract function for FeatureVector.

The extract function extracts a set of feature values based on relevance level
and feature.
Add StringHelper class.

The StringHelper class contains trim and split functions.