Add willAnswer stubbing method.

This allows more flexible stubbing using a block, similar to the doAnswer method in Mockito. What do you think?

I'm a bit unsure about exposing the NSInvocation directly. Maybe it should be encapsulated in an immutable wrapper similar to [InvocationOnMock](http://mockito.googlecode.com/svn/branches/1.7/javadoc/org/mockito/invocation/InvocationOnMock.html).