Fixup README code formatting.

- Spaces between method args.
- A space between `#` and the comment text.
Rename `Mock` to `Double` in class names, etc.

We've already removed `mock` as an alias for `double`,
might as well make class names and descriptions consistent.
Add changelog.
Declare some more classes as not part of the public API.
Merge pull request #462 from rspec/remove-mock-class

Remove mock class