Update custom fields

Hi Dale,

I added this based on an example php file, but it's an undocumented part of the API.  It's unclear what the API response should be.  I always got "1" in the data tag of the response.  I checked it against my version of Interspire and it is updating the data correctly.  

I also wrote a test ;)

This is my first ever pull request, so I'd appreciate any feedback you could give.

Thanks,
-Joe