Integration of voting system

## Added a vote system with the following

### Vote on question & vote on answer

 - Up and Down button
 - If already voted and vote again, back to neutral before next vote -means if you want to vote down after a vote Up, you have to click on a button, and then on Down
 - Votes are diplayed only on the question show page


### Tests for votes on question & answers
 - Test for vote up, down and for multi clicks votes
 - Factory created for further tests if needed

 
### Answer on answer system created
 - Created helper to define on which object an answer is posted
 - Added a field question_id to every answer for a fast search for all the answers of one question
 - Polymorphic_validation eisting function -which became useless with the created helper- removed
 - View NOT UPDATED to integrate comments of comments
 - Former answer question_id field NOT FILLED IN. A reset of DB or a dedicated method should be considered

### Bonus
Made tests on signup with valid id pass !!