RuboCop support

An initial attempt at adding RuboCop support (and sneak in a minor gemspec fix).

* It’s added only to the default Rake task, does not fail Travis builds (yet? I’m happy to make it fail builds…).
* Every fixed offence is applied in a separate commit, so I can quickly revert and whitelist it (many might be unacceptable on a pure taste basis and I’m perfectly fine with reverting them).
* No logic changes; certain offences marked as `FIXME` for fixing in subsequent refactorings.
* I tried to guess which offences are actually acceptable (and explain in the comments), but I’m happy to blacklist (and fix) any of them.
* Not sure what could be an acceptable line length (I love the good old 80, as it allows me to open two files side-by-side with a sixteen-point font, but many find it too short). Please advise. :)
When things don't fit I usually format the code differently to make it fit:

``` ruby
raise(
  NotImplementedError,
  "#{self.class}##{__method__} must be implemented"
)
```