Added basic commentable functionality.

- Comment is a polymorphic relation on Post. This allows us to add comments to any kind of resource we mark as commentable.
- Test the happy path for comment creation.
Extracted comment related HTML into paritals

- added comment creation form partial
- added comment list partial
Added unhappy path for blank comment creation.

- Added test
- Changed comment controller's save! to save. That was causing tests to blow up with an exception being thrown.
- Added validation for comment's content to not be blank
Updated post and comment UI
changed test helper's sign_in method to test_sign_in
Added tests and functionality to restrict comment posting to users who are logged in.
DRYed up adding comments tests
Added test and features to only show approved comments on the blog page.

- Test both that all users see approved comments, and that they dont see unapproved comments
- Added comments fixutre with polymorphic associations (commentable: one (Post))
- Added named scope :approved to Comment model for easier filtering