Feature-driven docs and testing for file and staging engines

Currently ADIOS has a lot of features, and not all of them are supported in all engines. It would be good to have a single location which tracks which features are supported by which engines and knows which tests require which features, that way we could drive testing so that we test that functionality on those engines, and drive the creation of a table in docs, a web page, or something similar that documents what engines actually run with those features.