Add Authors table to database

[seesarahcode]

• Create Authors table
• Add publication_id column - integer
• Add user_id column - integer
• Add byline column - string
• Add custom_name column - string
• Add is_editor column - boolean
• Add is_contributor column - boolean
• Authors table should:

has_many :publications, :through => :publication_authors
has_many :publication_authors

• Update Publications table:

has_many :authors, :through => :publication_authors
has_many :publication_authors

• Update PublicationAuthors table: belongs_to :authors
• Update factories
• Update unit tests to pass
• Update seed data