Add select2 for easy editing of tags

@jipiboily. 
Not sure if it's something we want or not (because of extra dependencies)
If yes, I will polish the PR
The idea is here the use of `select2 `to make the definition of tags easier (instead of comma separated, we have a better UI)

The only drawbacks is that it adds another dependency to select2-rails. 
UI is much more intuitive however

Let me know what you think. 
select2 is being used in spree with so far great success