Refactor specs

This PR will fix #430, and also does a bit of cleanup:

 * Don't explicitly define namespace modules in the spec – just include the namespace when calling `describe`.
 * The subject (in Cop specs) is always named "cop".
 * The spec offence messages are referenced by their literal string, not by the constant referring to said string.
 * Use `described_class` where possible.

The `favor_modifier_spec.rb` needed special attention, so please pay special attention to its diff.

This turned out to be a huge pull request, since I changed the indentation of most of the  specs… It's easiest to review each commit separately, and I have a feeling that maybe it should be split into 2 or 3 separate pull requests.