Merge pull request #11 from harvesthq/update-oauth-docs

Update OAuth docs
Link to OAuth 2.0 sample

Also: authentication -> authorization.
Updated register application link to say that you must be authenticated
Be more explicit about the meaning of `retainer_id` in invoice output

The presence of a `retainer_id` indicates that the invoice funded a retainer. It has no relation to whether the invoice is drawing from a retainer.
Merge pull request #18 from harvesthq/update_retainer_id_info

Be more explicit about the meaning of `retainer_id` in invoice output
Update retainer_id comment to be more explicit

Final change for https://github.com/harvesthq/api/pull/18
grammar
Merge pull request #19 from uniphil/patch-1

grammar