Better handling of UUID

[danschultzer]

Resolves #23

Currently, ExOAuth2Provider won't work if the UUID schema hasn't been compiled already. This proposed change will do the following:

1. Don't make an assumption of foreign_key_type for the resource owner, but instead allow for the foreign_key_type to be set in the config like so:

config :ex_oauth2_provider, ExOauth2Provider,
  resource_owner: {MyApp.User, :binary_id}

2. Built in UUID schema that can be used instead of provided schema. This can be set like so:

config :ex_oauth2_provider, ExOauth2Provider,
  resource_owner: {MyApp.User, :binary_id},
  app_schema: ExOauth2Provider.Schema.UUID

3. Explicitly state in readme that any customization will require that the Schema file is compiled first