Update git ignore file to ignore vim swp

Vim swp is generated when editing any file in Vim. It is unnecessary for
other people to see this file.
Add validation for artist id

Check if artist id has an existing artist. If it doesn't then the
validation will fail. This prevents the album model saving an album with
an invalid artist id.

Fixes issue #1. Note tests will fail because they don't currently
handle this validation.
Correct if statement so it returns true

The if statement to check for an existing artist would only return
false and never true. This meant that an album could not be saved even
with a valid artist id.

Fix this by adding an else branch that returns true if artist find by id
is not nil (false).
Get all rake tests working

The tests didn't work because the artist id was never 1. Make the test
suite pass by explicitly setting the artist id to 1 in the necessary
places.

The active record fixture website explains what happens when you don't
explicitly set an id:
http://api.rubyonrails.org/v3.2.8/classes/ActiveRecord/Fixtures.html
(see Stable, Autogenerated IDs).

- Add a new test to check that an album cannot be saved without a valid
  artist id.
- Reword testing the test for an empty artist id for readability.
Merge pull request #2 from jonathanyeong/fix_artist_id

Add validation to for artist id