Created tests and toggle functionality for private/public posts

Resolves #34 ... i think.
I would move this to the `spec/features/update_submission_spec.rb`. Rather than creating a list of submissions, probably just create one submission with the public flag already set to true:

```ruby
submission = FactoryGirl.create(:submission, user: user, public: true)

visit submission_path(submission)
click_button "Make Private"
```