libhdfs++: load configuration from files

One goal of the Configuration classes are to allow the consumers of the libhdfs++ library to deploy client applications into hadoop edge nodes and have them pick up the Hadoop configuration that has been deployed there.

Note that we also need to support the use case where the consumer application will manage Hadoop configuration files itself, or will handle all configuration out-of-band.

libhdfs++ should be able to read files that are found in the field and easily construct an instance that will communicate with the cluster.