change default solver of PCA to svd

[rasbt]

Description

Changed the default solver in PrincipalComponentAnalysis to 'svd' instead of 'eigen' to improve numerical stability.

Related issues or pull requests

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 @rasbt! Thanks for submitting the PR.

• There are no PEP8 issues in the file mlxtend/feature_extraction/principal_component_analysis.py !

[coveralls]

Coverage remained the same at 91.8% when pulling a61f61a on pca-pipeline into 93dee32 on master.