Tsam

* add second example which illustrates the output of the aggregation
* add the second publication about storage modeling in readme
* modify usable attributes to properties

- Improvement of extreme periods
- Move to MIT license

Patch to first public version with minor bug fixes

First public release
Features:
* different aggregation methods implemented (averaging, k-mean, exact k-medoid, hierarchical), which are based on scikit-learn or pyomo
* flexible integration of extreme periods as own cluster centers
* weighting for the case of multidimensional time-series to represent their relevance