2.2.0

• Support FastRGF as alpha version Please see https://github.com/fukatani/rgf_python/blob/master/FastRGF.rst
• Overall refactoring, Change directory configuration (Thank you for @StrikerRUS! )
• Enable to import by import rgf.RGFRegressor or other model. You can also use conventional import method import rgf.sklearn.RGFRegressor (Thank you for @StrikerRUS! )