FIX issue #4 ensuring content pointer is written

* uploadToContentService calls write() on the file object to ensure the
  cdncontent pointer is populated correctly
* Removed the assignment of CdnImage as ClassName as the extension method
  definitions get a bit mixed up otherwise. Instead, use the configuration
  option for changing the class used
Merge branch 'master' of git://github.com/silverstripe-australia/silverstripe-cdncontent