properly format computed property keys

new versions of esprima add a 'computed' field to Property nodes.

this patch adds the []'s in for them.

e.g. round-tripping the following JS through esprima and escodegen:

```js
var Test1 = {
  get [Symbol.creator]() {
  },
  get [Symbol.iterator]() {
  },
  set foo(v) { }
}
```

yields before:
```js
var Test1 = {
        get Symbol.creator() {
        },
        get Symbol.iterator() {
        },
        set foo(v) {
        }
    };
```

yields after:
```js
var Test1 = {
        get [Symbol.creator]() {
        },
        get [Symbol.iterator]() {
        },
        set foo(v) {
        }
    };
```