Define a generated name for self-generated root schema record

When an XML contains more than one element type, 
SchemaBuilder generates automatically a root record type on the
corresponding Avro schema. Generated root record type needs to be
named otherwise the Avro schema parsers would complain.

Avro schema type name is mandatory, according to specs.
Merge branch 'generated-schema-name-for-self-generated-root-record-type' of https://github.com/lucamilanesio/xml-avro into lucamilanesio-generated-schema-name-for-self-generated-root-record-type
Merge branch 'lucamilanesio-generated-schema-name-for-self-generated-root-record-type'