NaN value in dataframe caused _write_mysql error
- I have a dataframe with nan value inside and wanna use io.sql.write_frame function to store it in a table.
- But, nan value can not be treated well by MySQLdb, it will raise an exception:

``` python
OperationalError: (1054, "Unknown column 'nan' in 'field list'")
```
- I've tried to convert all nan value to None, but it turns out to be impossible,  fillna function treat param value=None as "the caller don't pass a value" not "the nan value should be None"
- I think add some hacks to convert nan to None in io.sql._write_mysql can fix this.
