Added unix socket support to MySQL adapter

I needed unix socket support to be able to use phinx with MAMP - all I've done is added the ability for a unix_socket argument to be passed along with all the other connection info:

    "environments" => array(
        "default_migration_table" => "migrations",
        "default_database" => "dev",
        "dev" => array(
            "adapter" => "mysql",
            "host"        => $_ENV['DB_HOST'],
            "name"        => $_ENV['DB_NAME'],
            "user"        => $_ENV['DB_USER'],
            "pass"        => $_ENV['DB_PASS'],
            "unix_socket" => $_ENV['DB_SOCK']
        )
    )