Refactored Converter to make it more extensible

- Minor refactorings to make the class more reutilizable

Made in association with: https://github.com/phpcr/phpcr-shell/pull/68
Moved the `SELECT` and `FROM` `expectToken` calls to here, as with the other commands. This enabled me to use the logic from `parseSource` for the `UPDATE` query in phpcr-shell.