Provide default namespace for schema and protocol.

[jangalinski]

Currently, we raise an exception when a top-level schema or protocol just provides a name, not a namespace.
We assume that it is practical impossible to use the schema or protocol (especially when it comes to code generation) without a full-qualified name (namespace + name).

Idea: We could set a default namespace on parser/declaration level that is used when an avro file does not define its own namespace. This would prevent the exception.