add max_len parameter to apriori

[rasbt]

Description

Adds a new max_len parameter for the frequent itemset generation via the apriori function to allow for early stopping.

Related issues or pull requests

Fixes #269

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 increased (+0.01%) to 90.858% when pulling 174b7e4 on max_len into edbca56 on master.