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...
Constructor argument to avoid creating the base directory if it doesn't exist and throwing an exception instead?