Add hspec-contrib with retryWith-function

Fix this https://github.com/hspec/hspec/pull/200.
Add hspec-contrib package and retryWith function.
Change hspec-core's exposed module for above package.


The reason for splitting `hspec` into multiple packages is so that the interface of the main `hspec` package becomes smaller, so that it is easier to keep it stable.  For this reason we should remove.  Eventually I also want to deprecate and remove most of the other modules, keeping only `Test.Hspec` (and maybe `Test.Hspec.Runner`).

For this reason we should not re-export `Test.Hspec.Contrib` here.  People who want to use the extra functionality can just depend on `hspec-contrib`.