Associate users and articles; require login to post

Resolves #12.
Since you added the `foreigner` gem, I'd include an `add_foreign_key` here:
```
def change
  add_column :articles, :user_id, :integer
  add_foreign_key :articles, :users
end
```
And if an `Article` should always have a `User`:
```
def change
  add_column :articles, :user_id, :integer, null: false
  add_foreign_key :articles, :users, dependent: :delete
end
```