change unstub warning to remove mention of expect
change unstub message to include replacemnt
Add specs showing how `and_call_original` can be used like `unstub`.
Tweak deprecation message a bit.

The deprecation is worded in general terms so it
seems odd for the replacement to be specific about
which message (particularly since it's not specific
about which object is passed to `allow`).
Merge pull request #486 from rspec/change_unstub_warning

Deprecation warning should mention that `unstub` has no replacement in new syntax