Added definitions for _(...)groupBy and toArray()

With lodash.d.ts v0.17 a call to ```groupBy``` followed by a call to ```toArray()``` before get the array value would not be recognized by TypeScript, causing the .ts compilation to fail.

Example:
```javascript
    _([{prop1: "a", prop2: "b"}]).groupBy("prop2").toArray().value();
```
With the added definitions the above code compiles succesfully. :-)