API change: metadata

In the current implementation the metadata infrastructure loosely follows the JavaBeans model. There's a drawback: it is not easily expandable, changing the property/feature set requires changing Java code, all the attributes and properties have to be coded separately. This is not elegant, and I'm too lazy to let it continue to be this way.

The metadata API will be simplified and will be accessible through setAttribute/getAttribute method pair. The code will be greatly simplified and unified, the API will become simpler to use, and the need to have a special metadata class may disappear altogether.

Coming in the next release. Existing model will be deprecated but still present for one more release, then it will be gone.