Feature/supporting UUID as primary key.

[siddhant3030]

To make this work you have to define certain things in config.exs file

config :spur,
  ecto_repos: [AppName.Repo],
  repo: AppName.Repo,
  audience_module: AppName.User,
  activities_table_name: "activity",
  uuid_primary_key_enabled: true,
  audience_assoc_name: :activities