Added possibility to change the namespace index

[ekmart]

Added possibility to change namespace index (default namespace index is 2)

Closes #1

// Fixed namespace index
client.NamespaceIndex = 3;

// From URI
client.SetNamespaceIndexFromUri("urn://N44/Ua/Device1");

// From identifier 
client.SetNamespaceIndexFromIdentifier("ua-server-identifier");