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

this fixes #532
@myronmarston How about:
By default, description, does not include the chained clauses.  
There would be another method called "include_chained_clauses" which would cause the chained clauses to be included in the description.  This could be clearly documented.

The matcher could look like:
```ruby
RSpec::Matchers.define :some_cool_matcher do
  chain(:foo) { }
  match { }
  include_chained_clauses
end
```