Move TopLevelDescribe to Rubocop::RSpec, so that cops folder is only cops
1.0.rc1 with thanks to @geniou!

* Update code to work with rubocop >= 0.19 (@geniou)
* Split `UnitSpecNaming` cop into `RSpecDescribeClass`,
  `RSpecDescribeMethod` and `RSpecFileName` and enabled them all by
  default. (@geniou)
* Add `RSpecExampleWording` cop to prevent to use of should at the
  beginning of the spec description. (@geniou)
* Fix `RSpecFileName` cop for non-class specs. (@geniou)
* Adapt `RSpecFileName` cop to commen naming convention and skip spec
  with multiple top level describes. (@geniou)
* Add `RSpecMultipleDescribes` cop to check for multiple top level
  describes. (@geniou)
* Add `RSpecDescribedClass` to promote the use of `described_class`.
  (@geniou)
* Add `RSpecInstanceVariable` cop to check for the usage of instance
  variables. (@geniou)