More flexible Tree generation (formerly the 'categories' idea)

[royerloic]

Right now the Tree is only three levels: (i) root, (ii) types and (iii) devices
It would be better if we could specify the 'path' in the tree in which we want to put a type,
this would allow for more flexible organization, a bit like package in Java.

To acheive this we could have a 'getPath' method in HalcyonNodeType
String getPath()

which would return strings like: 'root.hardware' or 'root.logging'
that specify where to put the items in the tree.