Merge pull request #114 from minimul/link-to-getting-started-guide

Add getting started guide to the README
Add valid_for_update? For Quickeebooks::Online::Model::Item
Almost non-sense test for sanity, ensure item responds to valid_for_update?
Merge pull request #115 from johnsonj/master

Item does not respond to valid_for_update? despite being an updatable model