Denormalize comments number into post

- Add commentsCount property to post variable to count the number of comments relating to that post
- Make sure all post instances will be initialized with 0 comments
- Add function to increment comment count by 1 every time that a new comment is posted
- Update Meteor project to 0.9 and update .id file of project
- Remove commentsCount variable from post_item template helper because it is now located in every post instance