Compression with C++ DataFile

There is no way to use compression with the C++ DataFileReader and C++ DataFileWriter, from what I can tell. Adding compression of the written blocks using boost streams is relatively straight forward and I can provide a patch if people are interested.

However, there are a couple caveats:

• the windows builds of boost don't currently include zlib support (required for compression) by default. You have to do extra work to get it.
• I don't know if doing it that way is compatible with other avro implementations