Support for vertex multiline label.

Still in progress.
Support for multiline labels in vertices ready.

Now the user can create vertices with multiline labes.
Read/Write support for files.

- A multiline label in a file is represented with a "\n".
- Changes has been made to handle new lines appropriately, for reading
  from and writing to files.
- Bug fix: Edge's label should be at the center of the two nodes.