Integrate rubocop

This is #78 rebased against master, squashed and taken a bit further.

I've also extracted a base rspec file as suggested by @JonRowe in #78; once it's reviewed by someone else I plan to move it into rspec-dev as part of the normal way we distribute shared build files.
We had, but rubocop was tripping up on the `private` declaration that was originally outside of the `if/else` (the deleted line 33):

```
Inspecting 23 files
...W...................

Offenses:

lib/rspec/support/encoded_string.rb:32:5: W: Useless private access modifier.
    private
    ^^^^^^^
```

...so I had to move it inside the if/else.  I thought it looked really odd to have:

``` ruby
if condition

private

  def bar
  end
end
```

...since in my mind something nested under an `if` should always be indented relative to it.

I'm fine with de-denting it though, but don't particularly care in this one case since it's so odd.