ENH: sql support for Timestamp (GH7103)

Closes #7103 

This adds a `pandas.io.sql.Timestamp` class to handle `Timestamp`s in `to_sql`. This basically converts it to a `datetime.datetime` before writing to the database.

Problem is that this makes it slower (I tested it with sqlite, and it's for a dataframe with only a datetime64 column about 20% slower), while it did already work before for some drivers (like psycopg2 and MySQLdb).