fix m2m between category and club

Originally there were 2 one-to-many relationships:
    1. A club has many category
    2. A category has many clubs

However, this did not correctly reflect many-to-many relationship between
categories and clubs.

Say Club 1 has category_id set to 1.
Say Category 1 has the club_id to something else.

When I get Category 1's clubs, Club 1 will be among the clubs. However
when I query Club 1's categories, Category 1 will not be there since
Category 1's club_id is set to something else.

The following resources were referenced to make the necessary changes
such that a many to many relationship between category and club exists:

http://railscasts.com/episodes/47-two-many-to-many
http://guides.rubyonrails.org/association_basics.html#the-has-many-through-association