Adding routes for signup, create post, read posts

*Adding route for user signup
users can be created using the REST API /signup

*Adding route for creating a new post
user can create a new post using the REST API /post
checking post tags for blank spaces and converting to py list

*Adding route to read a post using UUID
user can read a post by specifying the UUID
user can read it only if the post belongs to one of his groups

*Changing UserDAO contructor
removing the param app.sceretkey

*Updating unit tests to reflect the UserDAO constructor change

*converting the group_id of Post model to a string in __str__()