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.