Added support for values defined with Avro schemas

Hi Alexandre,

I have added basic support for values defined with the Avro schemas. The value will be translated to the string in JSON format. The user should be able to extract data from JSON using get_json_object UDF

https://cwiki.apache.org/confluence/display/Hive/LanguageManual+UDF#LanguageManualUDF-get_json_object

Please review the proposed changes when you have a chance.

Best regards,
Dmitry Vasilenko