Fixes from TraVarT-kconfig

[6b6279]

Most of these fixes came to be during the development of TraVarT-kconfig, which currently depends on my own fork.

In summary:

• Contain small Maven repository for FeatureIDE external library (see pom.xml and lib subdirectory)
• Add multiple addition methods for Constraints
• Fix retrieval of features in getFeature (fetch mapping from root feature to ensure that all features from the FM are reachable)
• Refactor IConfigurable (use “Configured” instead of “Selected”)
• Fix getGroup; now behaves according to Javadoc contract with regard to group indexes

It's a bit rough around the edges, but I'd be ready to refactor, e.g. the NPE bypass in setGroup.