Stop using env var for driver name configuration

[geertjanvdk]

Adding the environment variable to change the behavior of how driver is registered was a bit to impulsive. Instead, it was pointed out by @jackwilsdon on Gophers Slack to actually use a sub-package that registers the name "mysql".

package yourstuff

import (
    _ "github.com/golistic/pxmysql/mysql"
)

Much better.