Symbol IDs

[gsmetal]

Finally, I've found time to deal with this.

This PR based on #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.

class Sex < ActiveEnum::Base
  value :id => :m, :name => 'Male'
  value :id => :f, :name => 'Female'
end