add avro schema to support other languages

Add [Apache Avro](http://avro.apache.org/) protocol schema for other [Supported Languages](https://cwiki.apache.org/confluence/display/AVRO/Supported+Languages).It was only a partial modification to the attempt. Not centain if such avro schema can fit this project.

```
$ nosetests test/test_avroproto.py
....
----------------------------------------------------------------------
Ran 4 tests in 0.002s

OK
```
For better encapsulation, these constants should be fit inside a `setUpClass` method, like [that](https://github.com/OpenBazaar/OpenBazaar/blob/master/test/test_kbucket.py#L13). This is just a style suggestion, I don't mind if you leave it like that.