Add support to custom field on has_secure_password

[tinogomes]

Schema: Account(name:string, phrase:string)

class Account < ActiveRecord::Base
has_secure_password :password, :phrase
end

account = Account.create
account.errors[:phrase] # => ["can't be blank"]

account = Account.new(:name => "david", :password => "", :password_confirmation => "nomatch")
account.save # => false, password required
account.password = "mUc3m00RsqyRe"
account.save # => false, confirmation doesn't match
account.password_confirmation = "mUc3m00RsqyRe"
account.save # => true
account.authenticate("notright") # => false
account.authenticate("mUc3m00RsqyRe") # => account
Account.find_by_name("david").try(:authenticate, "notright") # => nil
Account.find_by_name("david").try(:authenticate, "mUc3m00RsqyRe") # => account

[dmathieu]

This feature has already been proposed and rejected by the core team. See #1440.

[tinogomes]

Sorry guys for this pull request. I haven't saw the previous discussions about it.