Add sfs cv generator message

[jrbourbeau]

Description

This PR adds a meaningful error message when a cross-validation generator is input to SequentialFeatureSelector

Related issues or pull requests

Fixes #373

Pull Request Checklist

• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• Added appropriate unit test functions in the ./mlxtend/*/tests directories (if applicable)
• Modify documentation in the corresponding Jupyter Notebook under mlxtend/docs/sources/ (if applicable)
• Ran nosetests ./mlxtend -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., nosetests ./mlxtend/classifier/tests/test_stacking_cv_classifier.py -sv)
• Checked for style issues by running flake8 ./mlxtend

[coveralls]

Coverage increased (+0.02%) to 90.924% when pulling 0511056 on jrbourbeau:add_sfs_cv_generator_message into 0f02c0d on rasbt:master.

[rasbt]

That looks great, thanks a lot!