Add pip installation method
Add pip installation method

This grants some advantages such being able to list the package in the requirements.txt and use it in continuous integration builds, also, it's a cleaner way.
dvro/UnbalancedDataset,fmfn/UnbalancedDataset,dvro/imbalanced-learn,dvro/imbalanced-learn,fmfn/UnbalancedDataset,dvro/UnbalancedDataset,scikit-learn-contrib/imbalanced-learn,scikit-learn-contrib/imbalanced-learn,glemaitre/UnbalancedDataset,glemaitre/UnbalancedDataset