ENH: sql support for Timestamp (GH7103)
Superceded by #8208 (converting to object dtype also converts the datetime64 values to datetime.datetime, so it is no longer needed that sqlalchemy can work with pandas' Timestamp type.

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Closes #7103, #7936

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).
