Introduces common DosomethingUserRemote abstract class.

#### What's this PR do?
- Splits `dosomething_uk_sso_user.inc` to fields common and UK-specific fields
- Creates new abstract class of a remote user in `dosomething_user` to extend other remote users from (e.g. Canada)

#### Testing
![screen shot 2014-12-03 at 12 40 55 am](https://cloud.githubusercontent.com/assets/672669/5272421/75eb2aac-7a85-11e4-8d76-74c1dcce14ef.png)
![image](https://cloud.githubusercontent.com/assets/672669/5272469/f13fce10-7a85-11e4-96cd-a059a891242c.png)

Potentially call this username? I'm just not crazy about having properties that seem to match with the Drupal User, but are named differently... but definitely open to hear the reasoning...