Add `add_token` method

- you may have been doing reads, then think 'oh no, I need to do writes', but how are you going to do writes? Your data-set instance doesn't have a token, does it? no.
Test that adding a token adds a token

- Nae validation on the token itself, however
set not add
Merge pull request #9 from alphagov/add_token_method

Add token method