Adding Namespace class to allow for betting handling of namespaces.

[redundent]

refs: #49

[asm0dey]

One small comment on implementation: when I'm creating a node with String.invoke() syntax I can't pass namespaces there cause attributes parameter accepts only pairs and should likely accept also something like Triple or Attribute

[redundent]

Fair point. I wanted to keep it simple with the existing Pair attribute logic but I can easily add another overload that will take in an object to allow more granular values.

[asm0dey]

My use case is creation of Atom feed. There are links with several attributes in different namespaces. Writing a block body looks a bit overwhelming for such a task. But generally the API works, thank you!

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