Use GUrl class instead of FILE type for reading and writing.

The use of the new GUrl class insread of the FILE type allows now for transparent input and output of an XML document in different URL types. So far only the file type URL exists (GUrlFile), but this can now easily be expanded to support piping into a string or direct http transfer. The latter is particularily useful for XML-RPC implementation.
Add file existence check.
Add GUrl usage information.
Implement URL string class.

The URL string class enables reading and writing into a string. Note that the printf() method has an internal buffer limitation to 8192 characters, and the scanf() method does not advanced the pointer, so it can't really be used for the moment.
Add GUrlString class.

The GUrlString class handles reading and writing into a string.
Correct comments.
Add testing of GUrlString class.
Correct comment.
Add GUrlString class.
Add Python interface for GUrlString class.
Remove file existence check from open() method.

The open() method should also work for files that do not yet exist (files in "w" mode), hence the file existence check has been removed.
Add unit test for GUrlString class.
Declare index parameters as const references.
Include <cstdio> header (required for fopen, etc).
Include <cstdio> header (required for fopen, etc).
Merge remote-tracking branch 'origin/integration' into integration