Add ability to select tags

Hey, 

I've added ability to select [tags](http://ivaynberg.github.io/select2/#tags) from select2. 

Instead of making a God-Method `select2` I'm suggesting to use separate method `select2_tag` :) 
If you agree with this convention I can split `select2` and `select2` with `search` field and add tests in next PR.