Add User.role and gem cancan
Finish admin delete topic feature

	1. add user model test about star unstar
	2. add fully controller test about topic
	3. add authorization about resourse topic
Several things:

  1. replace with github version shoulda to make more helpers
     available
  2. redirect user to root when user is not denied
  3. refactory ability to make it cleaner
  4. now penission can inherit, user < moderator < admin
Make model test better
remove topic controller test
Finish ability test about topic
Stuffs:
  1. add comments
  2. replace "delete" with icon
  3. clean code
Merge remote-tracking branch 'refs/remotes/origin/admin-delete-topic'

Conflicts:
	spec/models/reply_spec.rb
	spec/models/topic_spec.rb
	spec/models/user_spec.rb