Support generation of nested class structures

[jangalinski]

The great danger of single file code generation, as done by the default apache maven plugin, is that when you have the same internal schema re-defined (as re-use is forbidden by avro), you will get two generated files that overwrite/delete each other when they have the same namespace+name.

We started to work around this by making a single data class with nested inner clsases the default for generation from schema. However, now the inner classes have officially a different FQN then the FQN derived from the schema declaration, which led to bug #138 and issue #143.

That being said: It might still be useful to generated nested structures and we might pursue this further in future.