Implement a unix-like cat utility

Implement the basic functionality of "cat" and have it build as a separate executable.

2 Reasons for this:
We don't have any real integration tests at the moment so something simple to verify that the library actually works against a real cluster is useful.

Eventually I'll make more utilities like stat, mkdir etc. Once there are enough of them it will be simple to make a C++ implementation of the hadoop fs command line interface that doesn't take the latency hit of spinning up a JVM.