mysql-native: preview2

[Nick Sabalausky via Digitalmars-d-announce]

Made a couple more long-needed changes while I'm at it:

https://github.com/Abscissa/mysql-native-experimental
Tag: v0.2.0-preview2

- For better clarity, renamed `mysql.db.MysqlDB` to `mysql.pool.MySqlPool`.

- Package mysql.connection no longer acts as a package.d, publicly 
importing other modules. To import all of mysql-native, use `import mysql;`