Add ReactProps.func

This adds ReactProps.func so people don't need to write the
slightly-more-cryptic ReactProps.instanceOf(Function). We should have
had this all along.

src/core/ReactProps.js

@@ -73,6 +73,7 @@ var Props = {
 
   array: createPrimitiveTypeChecker('array'),
   bool: createPrimitiveTypeChecker('boolean'),
+  func: createPrimitiveTypeChecker('function'),
   number: createPrimitiveTypeChecker('number'),
   object: createPrimitiveTypeChecker('object'),
   string: createPrimitiveTypeChecker('string'),

src/core/__tests__/ReactProps-test.js

@@ -56,6 +56,7 @@ describe('Primitive Types', function() {
   it("should not throw for valid values", function() {
     expect(typeCheck(Props.array, [])).not.toThrow();
     expect(typeCheck(Props.bool, false)).not.toThrow();
+    expect(typeCheck(Props.func, function() {})).not.toThrow();
     expect(typeCheck(Props.number, 0)).not.toThrow();
     expect(typeCheck(Props.object, {})).not.toThrow();
     expect(typeCheck(Props.string, '')).not.toThrow();