Out-of-the-box Vertabelo XML format integration

This pull request contains 2 commits allowing jOOQ to integrate with Vertabelo. It gives the possibility to generate ORM by either:
* specifying Vertabelo XML file path, or
* downloading XML directly from the Vertabelo API.

The only thing to do to switch to the Vertabelo XML as a source is to change codegen configuration.

Note: The second commit (e65f62d) contains Base64Coder class - I'm not its author. It's licensed on the terms of many different licenses, in particular "Apache License, V2.0 or later" so it shouldn't be a problem to include it in jOOQ. I used this because I wanted to avoid adding a new dependency to your project (like Apache Commons Codec). Please feel free to change the base64 implementation.