ENH: add PostgreSQL and MySQL support for pandas.io.sql.get_schema

Simple changes to add PostgreSQL syntax to pandas.io.sql.get_schema. Added data type cases for get_sqltype() and a columns "flavor" for get_schema(). 

The logic of ``get_schema`` seems simple enough at the moment (though it still doesn't support Oracle). Should it be replaced by SQLAlchemy integration?

ref: https://github.com/pydata/pandas/issues/4163
old pr: https://github.com/pydata/pandas/pull/4574