invalid characters in schema for inner classes

Hey,
I'm generating Avro Schema for POJO that uses inner classes and because of that namespace field contains $, which is a character not allowed by Avro schema. I think substituting that with `.` is a good way to solve the issue.

`
{
          "type" : "record",
          "name" : "TestImpl",
          "namespace" : "package.Test$",
          "fields" : [  ...]
}
`
Thank you,
