UK SSO: Implement administration form.

#### What's this PR do?
- Provides system settings form for DoSomething UK OAuth integration settings 

#### How should this be manually tested?
- `drush en dosomething_uk`
- Set `dosomething_uk_oauth_url`, `dosomething_uk_oauth_key`, `dosomething_uk_oauth_secret` variables
- Clear the cache `drush cc all`
- Go to [modules administration](http://dev.dosomething.org:8888/admin/modules) and search for `UK`
- Click `Configure` button on `DoSomething UK` module
- It should bring you to [the form](http://dev.dosomething.org:8888/admin/config/dosomething/dosomething_uk):
  ![screen shot 2014-10-02 at 10 12 48 pm](https://cloud.githubusercontent.com/assets/672669/4496705/9074a3d2-4a68-11e4-90dc-d117a65a21bf.png)

#### What are the relevant tickets?
- Part of UK SSO refactoring: [#DS-377](https://jira.dosomething.org/browse/DS-377)

placeholder :pencil2: