Doc updates

First go at completing/ updating the client docs. Few questions/ notes etc:

* __Models:__ I've focused mainly on documenting the client object methods and haven't done a huge amount with the [models](https://github.com/mozilla/badgekit-api-client/blob/doc-updates/docs/models.md) stuff - _what I have done on that page was mostly guesswork_.. Should I add some examples and flesh the models info out more or would this be better suited to wiki type guides outside the `/docs`..?

* __Claim codes:__ Are we still using these? I've documented them in the same way as the rest.. However:
 * `claimClaimCode` didn't work for me - the `claimed` field seems to update in the db but my app hangs and submitted emails don't get added to the `email` field. (_I've commented out references to the method for the moment. Also didn't include method `claim` in [models](https://github.com/mozilla/badgekit-api-client/blob/doc-updates/docs/models.md) for the same reason._)
 * `updateClaimCode` gave me `[MethodNotAllowedError: PUT is not allowed]` - _commented out_.
 * `generateRandomClaimCode` gave me `[NotAuthorizedError: Missing JWT claim: body]` - _commented out_.
 * `getClaimCodes` without `badge` gives `[ResourceNotFoundError: /systems/badgekit/codes does not exist]` - should it be `[ContextError: Context not of required type: Badge]` ?

* On `updateBadgeInstance` I get `[MethodNotAllowedError: PUT is not allowed]` - _commented out for now_.

* On `deleteReview` the client seems to return null rather than the deleted review.. - _haven't included the response in the docs for now_.

* I've removed the `evidence` methods since I think these were removed from the API..

Give me a shout if anything isn't clear.. Feedback appreciated! :hear_no_evil: