added _.findKey definitions

lodash/lodash-tests.ts

@@ -546,6 +546,10 @@ var result = <any>_.cloneDeep(stoogesAges, function(value) {
 var foodDefaults = { 'name': 'apple' };
 result = <any>_.defaults(foodDefaults, { 'name': 'banana', 'type': 'fruit' });
 
+result = <string>_.findKey({ 'a': 1, 'b': 2, 'c': 3, 'd': 4 }, function(num) {
+  return num % 2 == 0;
+});
+
 var mergeNames = {
   'stooges': [
     { 'name': 'moe' },

lodash/lodash.d.ts

@@ -2206,6 +2206,35 @@ declare module _ {
 		object: any,
 		...sources: any[]): any;
 
+	/**
+	* This method is like _.findIndex except that it returns the key of the first element that 
+	* passes the callback check, instead of the element itself.
+	* @param object The object to search.
+	* @param callback The function called per iteration.
+	* @param thisArg The this binding of callback.
+	* @return The key of the found element, else undefined.
+	**/
+	export function findKey(
+		object: any,
+		callback: (value: any) => boolean,
+		thisArg?: any): string;
+
+	/**
+	* @see _.findKey
+	* @param pluckValue _.pluck style callback
+	**/
+	export function findKey(
+		object: any,
+		pluckValue: string): string;
+
+	/**
+	* @see _.findKey
+	* @param whereValue _.where style callback
+	**/
+	export function findKey(
+		object: any,
+		whereValue: Dictionary<any>): string;
+
 	/**
 	* Recursively merges own enumerable properties of the source object(s), that don't resolve 
 	* to undefined into the destination object. Subsequent sources will overwrite property