Provides an RSpec 3 compatible formatter.

* Opted to keep this separate rather than try to remove duplication
  with the RSpec 2 formatter. It's just subtlely different enough that
  trying to DRY this up would make the code harder to understand.
  little gain. It would also risk introducing bugs into the old
  formatter. Assuming this code is pretty much stable now and that any
  new features or bug fixes would apply only to this new version.
* ProgressFormatter is private, so now inherits from BaseTextFormatter.
  (Having a discussion with core team about whether it should be public.)
* Simplified `pending_fixed?` with new consistent API.
* Use `fixed_color` rather than removed `blue`.
* Use accessor on `execution_result` rather than treating it as a hash.

Fixes #13.
Fixes #14.

@tpope @JonRowe