making with concern and lazy-index fix
cleaner way to reindex data after save

Actually we don't need to have special variable to check if attribute has changed.
Mongoid API has this check.

As one additional advantage, we can use DSL form builder without configuring them.
For example RailsAdmin knows how to manage with the data type Array. 
update tests. One is failing
test
Merge pull request #1 from kritik/master

mongoid 3