Implemented new service interface.

The interfaces for the following services have been created:
- AnswerService
- QuestionService
- FeedbackService
- ReviewService
Changed domain/data model.

- Added IsPrivate field to question and answer.
- Added Title field to question.
- Added FeedbackToken field to QuestionEntity (a one-time toke used to
check if feedback came from the user that asked the question when this
user isn't authenticated).
- A question now has a Answer field instead of a Anser an Question field
and fixed all code that used this.
- Fixed some typos.
- Changed SourceType to SourceDefinition in Question and fixed all code
that used this.
- Made CreationTime a required field in the db.
- Added question field to feedback.
Added language definition.

- Added language definition to answer and question.
- Removed username and password from user, since we're going to use
openId.
Merge pull request #16 from teugis/master

Implemented new service interface.