Change id and file name to meet plugin guidelines

Author: Zamaroth

src/chart.js

@@ -52,7 +52,7 @@ var plugins = [];
 
 plugins.push(
     require('./plugins/plugin.filler.js')(Chart),
-    require('./plugins/plugin.gridLines.js')(Chart),
+    require('./plugins/plugin.gridlines.js')(Chart),
     require('./plugins/plugin.legend.js')(Chart),
     require('./plugins/plugin.title.js')(Chart)
 );

src/plugins/plugin.gridLines.js

@@ -270,7 +270,7 @@ module.exports = function(Chart) {
 	}
 
 	return {
-		id: 'gridLines',
+		id: 'gridlines',
 
 		beforeDatasetsDraw: function(chart) {
 			// Shut down the plugin if the chart is radar or polarArea