Change a way ActiveRecord's config is prepared for rake tasks

In commit d1d7c86d0 I moved setting migrations paths into activerecord's
railtie to remove Rails dependency on databases.rake. However, it
introduced a regression, ENGINE_PATH was not available at the moment, so
engine's migrations where not added properly to paths. Fix was added
at 97a4a771, but it changes a way things work from using ENGINE_PATH to
APP_RAKEFILE. Additionally, the config runs when the code loads, while
previously it ran in the db:load_config rake task

In order to make it more in pair with original version this commit
changes the config to run only on load_config task. This code uses the
fact that defining a task in rake does not overwrite, but only appends.

It also allows to get back to checking for ENGINE_PATH
Setup env and seed_loaded for DatabaseTasks outside load_config

Those vars can be used in tasks, which not call load_config.
Add `#any_of` query method to active_record

This method is inspired by [any_of from
mongoid](http://two.mongoid.org/docs/querying/criteria.html#any_of).

It allows to compute an `OR` like query that leverages AR's `#where` syntax:

    manual_removal = User.where(id: params[:users][:destroy_ids])
    users = User.any_of(manual_removal, "email like '%@example.com'", {banned: true}).destroy_all

It can be used directly on model class, or through an association, or
behind an other relation.

    manual_removal = User.where(id: params[:users][:destroy_ids])
    User.any_of(manual_removal, "email like '%@example.com'", {banned: true})
    @company.users.any_of(manual_removal, "email like '%@example.com'", {banned: true})
    User.where(offline: false).any_of( manual_removal, "email like '%@example.com'", {banned: true})

Its main purpose is to both :

* remove the need to write a sql string when we want an `OR`
* allow to write dynamic `OR` queries, which would be a pain with a string

There's also a negative version, `#none_of`. This will return all active
users :

    banned_users = User.where(banned: true)
    unconfirmed_users = User.where("confirmed_at IS NULL")
    active_users = User.none_of(banned_users, unconfirmed_users)