Add API documentation. Add script for generating API docs from service.php.

* Add summaries and descriptions to service.php
* Add script generate-api-docs.php, which will update API docs between markers based on the code in service.php.
* Run generate-api-docs.php with `php -f generate-api-docs.php`
* Include example PHP values for each type of parameter in generate-api-docs.php.
* Add links to relevant Basecamp API documentation in service.php. Include these when generating section in readme.
* Add link to service.php file in readme.
* Change `$service` in readme authorization example to `$client` to be consistent with API examples.