Extract rspec_reset/rspec_verify into a spec helper method.

This refactoring is in prep for removing rspec_reset/rspec_verify
from every object; by extracting it into a helper method it
will make it easier to change w/o breaking specs.
Externalize methods and state that was previously held on all objects.

- Remove `@mock_proxy` ivar
- Remove `rspec_verify`, `rspec_reset`, `__mock_proxy` and `__remove_mock_proxy` methods.

This is a refactoring that I'm doing to pave the way for the new expect
syntax (#153). As that syntax "wraps" objects in order to mock or stub
them, we need to externalize these methods and state. The methods directly
on the object (e.g. `stub` and `should_receive`) will simply delegate to
this external logic.

This refactoring had some nice side benefits:

- No need for the hacky YAML fix. It was only needed because we were storing
  `@mock_proxy` on the mocked object.
- AnyInstance no longer needs to much with `dup`; again, this was only needed
  because it dup'd the `@mock_proxy` ivar.
- The Marshal extension gets significantly simpler (again, due to not
  storing `@mock_proxy` on the object anymore).

Rather than storing the mock proxies on the objects, we are now storing
them in a hash, keyed by object_id.  This is pretty simple and consistent,
but could be problematic for objects that muck with `object_id`. That method
seems a bit sacred, though, so I'm not too worried about it.
Fix specs on 1.8.7.

Hashes are ordered on 1.9 but not on 1.8. Thus, we cannot count on
the order the mock proxies will be reset or verified since they are
held in a hash.
Make spec pending that intermittently fails on JRuby in 1.9 mode.

I've spent a few hours troubleshooting this and haven't been able
to make progress.  It's intermittent and bizarre.  For now, just
making it pending.
Rename `tracked_klasses` to `tracked_classes`.

As per @dchelimsky's suggestion.
RSpec::Mocks.space.mock_proxy_for => RSpec::Mocks.proxy_for

...for a bit shorter, nicer API.
Actually, this intermittently fails on JRuby 1.8, too.
Merge pull request #250 from rspec/externalize_methods_and_mock_state

Externalize methods and state that was previously held on all objects.