Add dynamic ".*_values_for" helpers

Add dynamic helpers for easy access to possible flag column values.

For example, this code

```ruby
class User < ActiveRecord::Base
  has_flags 1 => :smoke, 2 => :drink, :column => 'habits'
end
```

will add helper

```ruby
User.habit_values_for :drink
=> [2, 3]
```

I think it might be useful addition (i use it a lot for building Sphinx indices etc).