Search Scopes (Reloaded)

Using ransackable_scopes, users can define whitelists for allowed
model scopes.

Coded by @avit and @glebm, see #288 for the discussion.

As mentioned in [this comment](https://github.com/activerecord-hackery/ransack/pull/288#issuecomment-46663209), I have forked `glebm:search_scopes` and rebased it against `activerecord-hackery:master` as per today.

I'll repeat the rebase from time to time (or on request) until @jonatack or another collaborator can free some time to review this PR.