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

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