Version 0.12.0

We rapidly improved and added new functionalities in the past week. We also added the following features:

koalas:

• isna (#548)
• isnull (#548)
• notna (#548)
• notnull (#548)

koalas.DataFrame:

• bool (#533)
• reindex (#493)
• pivot (#532)
• transform (#541)
• median (#544)
• cumprod (#545)

koalas.Series:

• cummax (#534)
• cummin (#534)
• cumsum (#534)
• bool (#533)
• median (#540)
• transpose (#543)
• T (#543)
• cumprod (#545)
• hasnans (#547)

Along with the following improvements:

• Fix DataFrame.replace to take kdf.replace({0: 10, 1: 100}) (#527)