Implement a libhdfs(3) compatible API

Add a synchronous API that is compatible with the hdfs.h header used in libhdfs and libhdfs3. This will make it possible for projects using libhdfs/libhdfs3 to relink against libhdfspp with minimal changes.

This also provides a pure C interface that can be linked against projects that aren't built in C++11 mode for various reasons but use the same compiler. It also allows many other programming languages to access libhdfspp through builtin FFI interfaces.

The libhdfs API is very similar to the posix file API which makes it easier for programs built using posix filesystem calls to be modified to access HDFS.