Implemented directive for comments

created comments.html under templates directory.
directive is used to get comments, create comments, delete comments for both Question and Answers.
Since there are 2 separate services - one for Question and one for Answer - I couldn't make a general function for both types.
edit comments function in progress.
Finished editComment function

Note:
AnswerController still has functions for comments.
They are there for reference only; once comments directive polishing is done, delete those functions from AnswerController.
(There may be more things to polish in AnswerController, I don't have enough time to check them).