Check if playlist was found before loading songs

If there's no playlist loaded, an error was raised, so
we must check if the playlist have songs before create objects
Little refactoring

- Use LOAD_PATH instead of $:
- Add missing newline at the end of files
- Remove useless whitespace
Merge pull request #35 from PierreRambaud/playlist-verification

Check if playlist was found before loading songs
Merge pull request #36 from PierreRambaud/clean-code

Little refactoring
Integrate rubocop and begin the refactoring
Finish rubocop
Add rubocop in rakefile default task
Add travis
Change be_truthy to eq(true) for utils class
Merge pull request #38 from PierreRambaud/refactoring

Refactoring
Fix twitter link [ci-skip]