Added timeline model

Added a has_many association through timeline_user_permission between
user and timeline
Recorded new set of cassettes in order to pass tests after changing in
models
Added validation to check for presence of timeline title
Fixed Sadegh's issues
Added timeline controller
Fixed Sadegh's issues
Added TimelinePost model
Fixed Sadegh's issues
Added timeline_posts controller
Fixed some of Sadegh's issues
Recorded new VCR cassettes
Added admin? method to User and Timeline models

Fixed current_user_post method of TimelinePost model based on admin?
(Suggested by Sadegh)
Renamed timeline_posts controller to posts

Record new VCR cassettes for posts spec
Changed posts resources routing and added index action to post controller

Moved posts resources under timelines resources