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.

It seems at though you can only update one field at a time.  I tried a different xml pattern like the "add subscriber" custom fields, but my server only spit out errors.

It also became necessary to retrieve a subscriber ID so you could call the update function.  I used the "in_contact_list?" method, but changed the return values.

I also wrote some tests. ;)

This is my first ever pull request, so I'd appreciate any feedback you could give.

Thanks,
-Joe