add multiplexer function

[rasbt]

Description

Adds a new function, make_multiplexer_data that generates a n-bit Boolean multiplexer dataset for benchmarking and evaluating supervised machine learning algorithms.

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Related issues or pull requests

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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 remained the same at 90.795% when pulling f6fe517 on multiplexer into 5735e00 on master.

[coveralls]

Coverage remained the same at 90.795% when pulling 247c076 on multiplexer into 5735e00 on master.

[coveralls]

Coverage remained the same at 90.795% when pulling c82560c on multiplexer into 5735e00 on master.

[coveralls]

Coverage remained the same at 90.795% when pulling 19efe37 on multiplexer into 5735e00 on master.

[coveralls]

Coverage remained the same at 90.795% when pulling 33226d4 on multiplexer into 38d5964 on master.

[coveralls]

Coverage remained the same at 90.795% when pulling 33226d4 on multiplexer into 38d5964 on master.

[coveralls]

Coverage remained the same at 90.795% when pulling c68911f on multiplexer into 38d5964 on master.