Add Model.select/group/order/limit/joins/conditions/preload/eager_load class methods returning a lazy relation.

  Examples :

    posts = Post.select('id).order('name') # Returns a lazy relation
    posts.each {|p| puts p.id } # Fires "select id from posts order by name"

  Example :

    posts = Post.scoped
    posts.size # Fires "select count(*) from  posts" and returns the count
    posts.each {|p| puts p.name } # Fires "select * from posts" and loads post objects

methods to to_a.

This reverts commit f2c0725d.

This reverts commit 7cce95b2.

method_missing.

naming. Use bang methods convention in methods that alter the
relation.

hashes and arrays like #find does.