Support both mymysql and MySQL drivers; simplification of Count

* We now support these two drivers:
  https://github.com/ziutek/mymysql
  https://github.com/go-sql-driver/mysql
* Only mymysql has direct support for DATE (via mysql.Date),
  DATETIME and TIMESTAMP.
* Count has been simplified to always return int64 and don't
  require a type check