Simplify joinClasses utility, faster

`joinClasses` is only used for `createTransferStrategy` which only ever sends 2 arguments, `merge` is used the same way and also only supports 2 arguments. So I propose simplifying it and shaving a few bytes, it's (by itself) also considerably faster, not that it really matters, but it's something.

http://jsperf.com/joinclasses-args-vs-array/8

```
IE8: 331,053 => 808,776
IE11: 2,012,439 => 7,316,248
FireFox: 8,170,716 => 37,780,175
Chrome: 11,019,243 => 50,732,389
```

```
   raw     gz Compared to master @ 1b67ac90f2bdca189a61a3e873af157198cdf7a5
     =      = build/JSXTransformer.js
     =      = build/react-test.js
   -37    -50 build/react-with-addons.js
   -67    -32 build/react-with-addons.min.js
   -37    -50 build/react.js
   -67    -31 build/react.min.js
```