ENH: sql support for NaN/NaT conversions
UPDATE from @jorisvandenbossche:

Overview of current status writing nan values (see also tests added in #7100):
- For MySQL using pymysql/MySQLdb nothing works: you get message `Unknown column 'nan' in 'field list'` (see also eg http://stackoverflow.com/questions/23353732/python-pandas-write-to-sql-with-nan-values)
- Numeric columns:
  - working for sqlite and postgres
  - Only full NaN columns stay `None` in sqlite
- Object columns (eg strings)
  - for postgresql: NaN is converted to the string `u'NaN'`, which is not really what we want
  - for sqlite it is returned as `None`
- `NaT`:
  - postgresql: gives error on inserting "0001-255-255T00:00:00"
  - sqlite3: writing works, but reading it with query returns `'-001--1--1 -1:-1:-1.-00001'`
- MSSQL: not working with message "The supplied value is not a valid instance of data type float", see #8088 for more details

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not sure exactly what sql expects (Nones?) rather than np.nan (or NaT)

https://groups.google.com/forum/?fromgroups#!topic/pydata/lxhnFtuzvWQ

also provide pandas `datetime64[ns]`, instead of `datetime/date` types
https://github.com/pydata/pandas/issues/3532
