Add subcommand to divide data into training set and test set

[ysk24ok]

Add subcommand (like cv_split) to divide data into training set and test set for cross validation.

After converting MovieLens data to cpmf format,

$ python scripts/convert_movielens.py PATH/ml-1m/ratings.dat --separator :: > input/ml-1m

users will split the data into training set and test set by the following command.

$ ./mf cv_split input/ml-1m --training_size 0.8

or

$ ./mf cv_split input/ml-1m --test_size 0.2