[WIP] Adding a module for photometric image quality measures

[vmirly]

Description

A module for photometric image quality measure

• Contrast
• Brightness
• Focus and sharpness
• Illumination
• Face image quality index (FQI) (Ref: proposed in here)

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

[pep8speaks]

Hello @vmirly! Thanks for submitting the PR.

• There are no PEP8 issues in the file mlxtend/image/photometric.py !

[coveralls]

Coverage remained the same at 91.619% when pulling 5debbf1 on vmirly:vm-photometric into cfc4123 on rasbt:master.