Implement a HDFS filesystem using the WebHDFS API

I've implemented a PyFS class that supports all methods, via the speedy WebHDFS API available in Hadoop 2. This can be a preferable alternative to using some of the available C libraries for interacting with HDFS or using a subprocess to `hadoop fs -ls /` and parse the output which can be very slow.

It's still WIP and needs more testing, I don't think it fully passes all of the PyFS unit tests yet, but it will do soon enough. Thoughts welcome...
You've got `update` in there twice, which throws an error.