added blog functionality

[Pivotal tracker blog feature] (https://www.pivotaltracker.com/story/show/68944882)
1. Created posts & comments MVC
*Post has author, title, content & date
*Comment has name, body & references a post_id
2. Admin only can create & update posts. Any user can create a comment.
3. Added blog link to footer
4. Basic HTTP authentication required to create a new blog post with username = admin & password = secret
5. Created unit tests for Rspec models folder files
6. Created Acceptance tests for Rspec features folder files