Added Songs and Chords models

* Songs and Chords have a has_and_belongs_to_many relationship to one another
* Added validations and null: false requirements to Chords and Songs
* Removed comments, changed single quotes in simple_form initializer
* Removed comments from db/seeds.rb
* Removed Factory Girl and Rspec rails gems
Merge pull request #5 from adwns42/add-chords-and-songs-models

Added Songs and Chords models