Simply SCrypt

Because `devise-encryptable` + `devise-scrypt` + `scrypt` currently doesn't work and are too many moving/incompatible parts, this is patch makes it far simpler to choose from the two best available secure hash engines available.  Note: the `scrypt` gem is stable but seeking a new maintainer.

For a project without existing users, this works:

1. Ensure these are in your Gemfile
```ruby

    gem 'devise' # or :git => 'https://github.com/steakknife/devise.git', :branch => 'feature__proper_scrypt' # for now
    gem 'scrypt'
```
2. Don't use `devise-encryptable`
3. Remove `:encryptable` from the User model
4. Set `config.encryptor = :scrypt` in config/initializers/devise.rb
5. Optionally set SCrypt settings
```ruby
    # config/initializers/scrypt.rb
    SCrypt::Engine::DEFAULTS[:key_len]     = 128
    SCrypt::Engine::DEFAULTS[:salt_size]   = 32
    SCrypt::Engine::DEFAULTS[:max_mem]     = 4*1024*1024
    SCrypt::Engine::DEFAULTS[:max_time]    = 0.4
    SCrypt::Engine::DEFAULTS[:max_memfrac] = 0.3
```
6a. Create a new migration `rails g migration DeviseScryptPasswordLength`
6b. Drop this into your project's migrations:
```ruby
    # db/migrate/20130331100000_devise_scrypt_password_length.rb
   class DeviseScryptPasswordLength < ActiveRecord::Migration  
      def up  
        change_column :users, :encrypted_password, :text
      end
      def down
        change_column :users, :encrypted_password, :string
      end
    end
```

Note: This does not convert passwords using Bcrypt or anything else.  It's only for projects without any existing users.