Message expect/allow API independent of configurable syntax.

An API independent of configurable syntax for setting message expectations/allowances is necessary, as highlighted by rspec/rspec-rails#764 and rspec/rspec-mocks#306.

Proposed interface by @myronmarston in rspec/rspec-rails#764:

```ruby
RSpec::Mocks.expect_message(object, :message).with("foo").and_return(5)
RSpec::Mocks.allow_message(object, :message).with("foo").and_return(5)
```

I'll upgrade this to a pull request once I've written code at Rails Camp this weekend.