Export (Transaction).tx *sqlx.Tx field

This PR exports the [`(Transaction).tx *sqlx.Tx`](https://sourcegraph.com/github.com/jmoiron/modl@master/.GoPackage/github.com/jmoiron/modl/.def/Transaction/tx) field.

We use an internal fork of modl at @sourcegraph with this field exported. It makes it easier to add additional functionality to `SqlExecutor` that makes use of `sqlx` and `database/sql` methods/helpers. Here 2 examples from our codebase where exporting this field has been necessary/helpful. I'm just mentioning these because this is essentially an API design decision and real world use cases are often helpful to me in these situations.

1. We have a Copy function that takes an `sqlx.Tx` (and calls `.Prepare` on it), but internally all of our DB handles are `modl.SqlExecutor`:

```go
// Copy executes a PostgreSQL COPY statement. All of the rows must have the same
// data type, which must be mapped to a table using modl.
func Copy(tx *sqlx.Tx, tableName string, rows interface{}, omitCols ...int) error {
   ...
}
```

In order to be able to call `Copy` on a `modl.SqlExecutor` or `*modl.Transaction`, the `tx` field must be exported. Then we can call, e.g., ` db.Copy(dbh.(*modl.Transaction).Tx, "mytable", data)`.

2. We have helper functions for selecting a single SQL result into a variable, which accepts a `modl.SqlExecutor` as an argument and wants to be able to call `QueryRow` on the DBMap or Transaction:

```go
func selectVal(e modl.SqlExecutor, v interface{}, query string, args ...interface{}) error {
	e = getUnderlyingSQLExecutor(e)

	var row *sql.Row
	switch e := e.(type) {
	case *modl.DbMap:
		row = e.Dbx.QueryRow(query, args...)
	case *modl.Transaction:
		row = e.Tx.QueryRow(query, args...)
	default:
		panic("selectVal: unknown type: " + reflect.TypeOf(e).String())
	}

	if row != nil {
		err := row.Scan(v)
		if err != nil {
			return err
		}
	}

	return nil
}
```

The `tx` field must be exported in order to be able to use sqlx's `QueryRow` on a `*modl.Transaction`. Note that the analogous `Dbx` field on `DbMap` is exported, so it would seem consistent for `Tx` to also be exported.