Ce 1019 exacttarget update user part3 ext refactoring

Likewise, these are the only two lines different than `ExactTargetApiDataExtension`,  and the other methods could probably be generalised too. It's possibly better to just have a generic API class that does the create, update, delete, retrieve, and the necessary wrapping of the passed variables, and move the slight differences in data preparation elsewhere. What do you think?