bike-sharing: split test/train data in CV based on day, other minor changes.

* Split test/train data for CV according to day number, as to use the
  same split between test data / public leaderboard and get more faithful
  CV scores;
* Change command line arguments accordingly;
* Round results before converting them to integers;
* Do not convert pandas dataframes to numpy arrays in preprocessing,
  not needed (automatically done by the estimator).