Add an overload of doReturn with a varargs signature.

Adds the ability to write doReturn(a, b, c).when(...) to match the existing when(...).thenReturn(a, b, c). The current syntax is doReturn(a).doReturn(b).doReturn(c).when(...).

Feature request for this is at https://github.com/mockito/mockito/issues/118
Discussion on Google Groups is  at https://groups.google.com/forum/#!topic/mockito/dHioyTNJ-ZM (linked from the feature request).