Namespacing helper methods in TopNode

Created the following, with tests, in TopNode:

- hasNamespace
- getNamespace
- getNamespaces
- getNamespaceNames

They do pretty much exactly what you think they do. :)

FYI -- this relates to commit 5ecd82f in master, which allows TopNodes to be created with an optional namespace (I put it right in master since it's a pure convenience thing, with a test).