adds support for xml based serialization and deserialization

[emazv72]

This PR basically adds support for an alternative - xml based - format for serialization and deserialization.

The package MigraDoc.DocumentObjectModel.IO.Xml has been added for parsing.
In order to support serialization, a new XMLSerializer has also been added.

Some sample code follows:

#if DEBUG

		MigraDoc.DocumentObjectModel.IO.Xml.DdlWriter.WriteToFile(document, "MigraDoc.xml");

		using (StreamReader sr = File.OpenText("MigraDoc.xml"))
		{
			var errors = new MigraDoc.DocumentObjectModel.IO.DdlReaderErrors();
			var reader = new MigraDoc.DocumentObjectModel.IO.Xml.DdlReader(sr, errors);

			document = reader.ReadDocument();

			using (StreamWriter sw = new StreamWriter("MigraDoc.xml.errors"))
			{
				foreach (MigraDoc.DocumentObjectModel.IO.DdlReaderError error in errors)
				{
					sw.WriteLine("{0}:{1} {2} {3}", error.SourceLine, error.SourceColumn, error.ErrorLevel, error.ErrorMessage);

				}

			}

		}

#endif