Adds an Opportunity

If you have trouble with creating an Opportunity, try creating Opportunities via the web interface, and then access those Opportunities via the API. This way you can see examples of the fields and sub-elements attached to the Opportunity. A common source of problems during write/update request is caused when users omit required fields, or insert invalid data in a field (e.g. reference a CATEGORY_ID that does not exist in the user's Insightly instance.