Initial support for Avro.

The type mapping between Avro types and Parquet types is working, as well as read and write support, including Hadoop input and output formats.

Still outstanding:

* Support fixed byte types (FIXED_LEN_BYTE_ARRAY is not yet supported generally).
* Nested records do not work. For some reason the start() method on AvroGenericRecordConverter for the nested record is not being called from the read path, but I haven't been able to debug this. (Any help appreciated.)
* Support specific and reflect Avro types.

Cheers,
Tom