Symbol IDs

Finally, I've found time to deal with this. 

This PR based on https://github.com/adzap/active_enum/pull/28

I completely implemented logic: if ids are symbols, names are not symbols. 
Added some restrictions of using mixed id types in one enum and added tests.

```ruby
class Sex < ActiveEnum::Base
  value :id => :m, :name => 'Male'
  value :id => :f, :name => 'Female'
end
```