raise notfittederror in stacking estimators

[rasbt]

Description

Make sure that all stacking estimators raise NonFittedErrors when an inference method is called prior to fit.

Related issues or pull requests

Fixes #289

Pull Request requirements

• Added appropriate unit test functions in the ./mlxtend/*/tests directories
• Ran nosetests ./mlxtend -sv and make sure that all unit tests pass
• Checked the test coverage by running nosetests ./mlxtend --with-coverage
• Checked for style issues by running flake8 ./mlxtend
• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file
• Modify documentation in the appropriate location under mlxtend/docs/sources/ (optional)
• Checked that the Travis-CI build passed at https://travis-ci.org/rasbt/mlxtend

[coveralls]

Coverage decreased (-0.02%) to 91.728% when pulling 8fed8ee on nonfitted into bac2d26 on master.