Add NamespaceCommentCheck to the Google module.

Summary:
This uses a bit hacky way to set the defaults for the spaces before
comments, but it's also one of the simplest ways. Fixed a bug with how the
SpacesBeforeComments option was used.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5410