add chained expectation descriptions to the default descriptions of custom matchers with fluent interfaces

this fixes #532
Hmm, that's certainly one idea.  I'm concerned that it would create confusion when the user starts with this:

``` ruby
RSpec::Matchers.define :some_cool_matcher do
  chain(:foo) { }
  match { }
end
```

...and then, after noticing the `description` and thinking they want to append a bit to what it was already producing, they change it to:

``` ruby
RSpec::Matchers.define :some_cool_matcher do
  chain(:foo) { }
  match { }
  description do
    super() + " my extra"
  end
end
```

...expecting the `super()` there to get the same description that was already being produced.  It would be confusing for it to _not_ include the chained bits when `description` did previously.