Allow labels to take on integer values represented by the string type.

[raj-sinha]

Allow labels to take on integer values represented by the string type.

This change allows the input label type to be a string containg an integer, as well as the empty string which is mapped to the unlabeled data value.

This feature is supported and tested only for the CSV data loader.

Summary:

1. dtype of label column will be str or int.
2. empty string is a legal value.
3. empty string will be internally mapped to the unlabeled value.
4. internally the map is: {'1': 1, '0': 0, '': -1, '-1': -1}.