Version 0.15.0

We rapidly improved and added new functionalities, especially for groupby-related functionalities, in the past weeks. We also added the following features:

koalas.groupby.GroupBy:

• size() (#593)
• filter() (#614)
• cummax() (#610)
• cummin() (#610)
• cumsum() (#610)
• cumprod() (#610)
• rand() (#619)

koalas.groupby.SeriesGroupBy:

• apply() (#609)
• value_counts() (#613)

koalas.indexes.Index:

• size() (#623)

Along with the following improvements:

• Add multiple aggregations on a single column (#602)
• Add axis=columns to count, var, std, max, sum, min, kurtosis, skew and mean in DataFrame (#605)
• Add Spark DDL formatted string support in read_csv(names=...) (#604)
• Support names of index levels (#621, #629)
• Add as_index argument to groupby. (#627)
• Fix issues related to multi-index column access (#594, #597, #606, #611, #612, #620)