Add xio_gdal_handler

[davidbrochart]

In addition to stream-like objects, this PR gives support for FILE-like objects to format handlers.
This first commit only changes the xio_binary format. I'll update the other formats after review.
See #132.

[davidbrochart]

Now the file and vsilfile classes are pretty close to a stream. I think we should just have a "wrapper" class for file-like objects and stream-like objects, that should implement these common methods:

• read(std::string& buffer)
• write(std::string& buffer, std::size_t size)

And get rid of the "overload" dump_bin_imp and load_bin_imp functions.

[davidbrochart]

The stream and file wrapper classes implement the methods needed in the compression formats that we currently have, so they are compatible with a stream or a file type.
If a future compression format needs a proper stream or file object (not our wrapper), we could specialize the load/dump functions for this particular type (e.g. stream) and get the object from our wrapper, and throw an "unsupported format" for the other type (e.g. file), like "GDAL doesn't support this format (because GDAL is a file-based IO handler, and the format is stream-based)".
This is the best I can think of, since wrapping a file into a stream doesn't seem to be a trivial task.

[davidbrochart]

Not sure about the name xio_gdal_handler. GDAL's documentation talks about Virtual File System, and the functions are called VSI functions. But neither xio_vfs_handler or xio_vsi_handler sound good to me.

[JohanMabille]

LGTM!

I could be nice to do a pass on API homogeneity, but this can be done in a dedicated PR!