Allow symbol as object keys

**TypeScript Version:** 
1.8.10 - editing with github's Atom / TS

**Code**

``` ts
// A self-contained demonstration of the problem follows...
export interface Container {
  stores: { [s: symbol]: string };
}
```

**Expected behavior:**
It should be fine ...

**Actual behavior:** 
err: `An index signature parameter type must be 'string' or 'number'`
