[Spec::Linter] Check for comments at the top of specs

Closes CocoaPods/CocoaPods#1192
[Spec DSL] Fix doc example to use actual attribute.
[Specification::DSL] Clarify that the documentation attribute is not used anymore

See #20
[Specification::DSL] Deprecate documentation attribute

Closes #20
[Specification::Linter] Give it a break with the punctuation thing!

Closes CocoaPods/CocoaPods#1242
Merge branch 'master' into feature-frameworks-bundles

* master:
  [Specification::Linter] Give it a break with the punctuation thing!
  [Specification::DSL] Deprecate documentation attribute
  [Specification::DSL] Clarify that the documentation attribute is not used anymore
  [Spec DSL] Fix doc example to use actual attribute.
  [Spec::Linter] Check for comments at the top of specs
[Specification::DSL] Fix resource_bundles documentation