read_sql should convert Numeric to decimals
`pd.read_sql` converts columns of type `sqlalchemy.Numeric` to floats as the below selection appears to suggest. This seems counter intuitive since in the `NUMERIC` and `DECIMAL` types are interchangeable. Also the `NUMERIC` type has an option `asdecimal` that defaults to true (http://docs.sqlalchemy.org/en/latest/core/type_basics.html#sqlalchemy.types.Numeric). I would expect that `sqlalchemy.Numeric` types get converted to `decimal.Decimal`.

https://github.com/pandas-dev/pandas/blob/master/pandas/io/sql.py#L1187-L1190
