function makeXmlMasterRel(masterSlideObject) {
		var relCount = masterSlideObject.rels.length
		var defaultRels = gObjPptx.slideLayouts.map(function(layoutDef, idx) {
			return { target: '../slideLayouts/slideLayout'+ (idx + 1) +'.xml', type: 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/slideLayout' };
		});
		defaultRels.push({ target: '../theme/theme1.xml', type: 'http://schemas.openxmlformats.org/officeDocument/2006/relationships/theme' });

		return gObjPptxGenerators.slideObjectRelationsToXml(
			masterSlideObject,
			defaultRels
		);
	}
Generates XML string for the master file.
@param {Object} masterSlideObject slide object
@return {String} complete XML string ready to be saved as a file