Create a model, a DAO, a xml mapping and etc.

1. Model (domain) changes
1.1 Adding the following new classes: Voting, VotingOption.
1.2 Adding new field voting:Voting to Topic.
2. DAO layer changes
2.1 Adding XML mapping for model changes
2.2 Adding migration for model changes
2.3 Adding VotingDao, VotingOptionDao interfaces and their implementations
2.4 Adding unit-tests for model changes (CRUD)