Add data validity

Providing dropdown list validation
Example: 
ws.setValidation({
    type: "list",
    allowBlank: 1,
    showInputMessage: 1,
    showErrorMessage: 1,
    sqref: "X2:X10",
    formulas: [
      'value1,value2'
    ]
  });

lib/WorkSheet.js

@@ -71,6 +71,30 @@ WorkSheet.prototype.Image = Image.Image;
 WorkSheet.prototype.getCell = getCell;
 WorkSheet.prototype.setWSOpts = setWSOpts;
 WorkSheet.prototype.toXML = toXML;
+WorkSheet.prototype.setValidation = setValidation;
+
+var countValidation = 0;
+function setValidation(data) {
+	countValidation++;
+    this.sheet.dataValidations = this.sheet.dataValidations || {};
+    this.sheet.dataValidations['@count'] = countValidation;
+
+    this.sheet.dataValidations['#list'] = this.sheet.dataValidations['#list'] || [];
+    var dataValidation = {};
+    Object.keys(data).forEach(function (key) {
+        if (key == 'formulas') { return; }
+
+        dataValidation['@' + key] = data[key];
+    });
+    data.formulas.forEach(function (f, i) {
+        f = typeof f == 'number' ? f : '"' + f + '"';
+        dataValidation['formula' + (i + 1)] = f;
+    });
+    this.sheet.dataValidations['#list'].push({
+        dataValidation: dataValidation
+    });
+}
+
 
 function getCell(a,b){
 	var props = {};
@@ -301,6 +325,7 @@ function toXML(){
 		'sheetData',
 		'autoFilter',
 		'mergeCells',
+		'dataValidations',
 		'printOptions',
 		'pageMargins',
 		'drawing'
@@ -326,4 +351,4 @@ function toXML(){
 }
 
 
-module.exports = WorkSheet;
+module.exports = WorkSheet;