Support json has_many though relationships

[schpet]

Supports two types of has_many through:

1) C has a b_id, B has a a_id

    class A < AR::Base
      has_many :bs
      has_many :cs, through: bs
    end

    class B < AR::Base
      belongs_to :a
      has_many :cs
    end

    class C < AR::Base
      belongs_to :b
    end

2) B acts as a join table, similar to HABTM

    class A < AR::Base
      has_many :bs
      has_many :cs, through: bs
    end

    class B < AR::Base
      belongs_to :a
      belongs_to :c
    end

    class C < AR::Base
    end

Fixes #15

[schpet]

Really appreciate this library! Was having trouble figuring out how to directly serve json from postgres and this really helped me out. Please let me know if there's anything I can add or change with this, it's my first time dipping into AR stuff 😨.

A big thanks, and happy October 🎃

[jackc]

Thanks! Looks good. I merged and released a new gem.