Allow Model#get to accept multiple arguments

Finally growing frustrated with seeing code blocks like this

```javascript
var a = book.get('a');
var b = book.get('b');
var c = book.get('c');

doSomething(a);
doSomething(b);
doSomething(c);
```
in my code, I decided to write up a quick pull to allow this beautifulness instead:

```javascript
_.each(book.get('a', 'b', 'c'), doSomething);
```

or just

```javascript
var letters = book.get('a', 'b', 'c');
console.log(letters); // ['foo', 'bar', 'baz']
```

This will give Model#get closer feature parity with Model#set, which can take multiple attribute/value pairs as arguments to assign.