Support @Exclude of specs

Originally reported on Google Code with ID 236

```
An @Excluded spec shouldn't have a @RunWith annotation so that it won't normally be
run. EmbeddedSpecRunner should compensate for that with a Request that returns Sputnik.
This should allow to write most embedded specs as regular classes instead of strings.

```

Reported by `pniederw` on 2012-02-26 22:10:23
