Fix - features: Length links wasn't operating correctly with an initial default page length of -1 (all rows)

- DataTables uses -1 as the infinite page length number, so having a
  default of -1 in this plug-in causes a conflict.
- This fixes #210 - thanks `malte-witt` for the test case and bug report

features/lengthLinks/dataTables.lengthLinks.js

@@ -37,7 +37,7 @@ $.fn.dataTable.LengthLinks = function ( inst ) {
 	var api = new $.fn.dataTable.Api( inst );
 	var settings = api.settings()[0];
 	var container = $('<div></div>').addClass( settings.oClasses.sLength );
-	var lastLength = -1;
+	var lastLength = null;
 
 	// API so the feature wrapper can return the node to insert
 	this.container = function () {