Timestamp sub second resolution not supported with MySQL
Hello,

Timestamp sub second resolution doesn't seem to be supported with mysqldb.

See:

```python
#!/usr/bin/env python
# -*- coding: utf-8 -*-

import pandas as pd
import numpy as np
import sqlalchemy

def main():
    N = 5
    df = pd.DataFrame({'Date':pd.date_range('2000-12-29', freq='D', periods=N), 'Open': range(N)})
    scheme = 'mysql+mysqldb'

    config = {
        'host'    : 'localhost',
        'database': 'test',
        'user'    : 'root',
        'password': '123456',
        'port': 3306
    }

    db_uri = "{scheme}://{user}:{password}@{host}:{port}/{database}"
    db_uri = db_uri.format(
        scheme = scheme,
        user = config['user'],
        password = config['password'],
        port = config['port'],
        host = config['host'],
        database = config['database']
    )
    engine = sqlalchemy.create_engine(db_uri)

    df["Date"] = df["Date"] + np.timedelta64(1, 'us') # to test resolution issue

    df = df.set_index("Date")

    print(df)
    print(df.dtypes)
    print(type(df.index), df.index.dtype)
    print(type(df.index[0]))

    df.to_sql("orcl", engine, flavor="mysql", if_exists="replace")

if __name__ == '__main__':
    main()
```

doesn't raise any error but when I queried database I noticed that I have only second resolution timestamp (no microsecond, in fact no fractional seconds).

```
$ mysql -u root mysql -p
Enter password:
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 40
Server version: 5.5.38-0ubuntu0.14.04.1 (Ubuntu)

Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> select * from orcl;
+---------------------+------+
| Date                | Open |
+---------------------+------+
| 2000-12-29 00:00:00 |    0 |
| 2000-12-30 00:00:00 |    1 |
| 2000-12-31 00:00:00 |    2 |
| 2001-01-01 00:00:00 |    3 |
| 2001-01-02 00:00:00 |    4 |
+---------------------+------+
5 rows in set (0.00 sec)

mysql> show columns from orcl;
+-------+------------+------+-----+---------+-------+
| Field | Type       | Null | Key | Default | Extra |
+-------+------------+------+-----+---------+-------+
| Date  | datetime   | YES  | MUL | NULL    |       |
| Open  | bigint(20) | YES  |     | NULL    |       |
+-------+------------+------+-----+---------+-------+
2 rows in set (0.00 sec)
```

Maybe storing `datetime64` into integer value could be the most portable solution over database, driver...
