Fix issues with generation of ISO-8601 date strings with UK locale + 24 hours disabled on device

If you switch to the United Kingdom locale and then disable the 24 hour clock RestKit will emit date strings such as "2013-06-10T11:55:00 PMZ"

* Add locale option to `RKISO8601DateFormatter` to enable you to specify the locale for generation.
* Update preferred date formatter to use `en_US_POSIX` locale to ensure dates generated are in UTC 24 hour notation
* Do a bit of cleanup to the ISO-8601 date formatter. It's quite a mess in there.