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Provide default xml namespace if none specified
Delegate schema to module, not namespace

This prevents errors with non-namespaced XML structures (such as RSS,
OPML, etc)
Spec improvements and simplifications
Move schema declaration to Structure module
Extract property definitions for Structure module
Remove useless Xommelier::Xml::Element#root method
Use Structure::Property objects for serializing/deserializing
Add Xommelier::RSS family
Initial Xommelier::OPML implementation
Bump patch version to 0.1.17