TST: sql add more tests (NaN handling, ..)

This PR just adds some tests, not fixes to get them all running.

Summary:
- NaN writing not working for MySQL (or pymysql), but does work for sqlite and postgresql
- NaN reading does work for float columns. But not for string columns (converted to None or u'NaN') or for sqlite full float columns
- NaT in datetime columns only works for sqlite for writing. Writing postgresql gives an error on inserting the date `0001-255-255T00:00:00`.